Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements - Fair Value Measurements By Level for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)

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Fair Value Measurements - Fair Value Measurements By Level for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Assets    
Cash and cash equivalents $ 12,123 $ 5,342
Investments available for sale:    
Total assets at fair value 20,559 12,965
Liabilities    
Total liabilities at fair value 11 95
U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments available for sale:    
Fair Value 212 361
Corporate securities    
Investments available for sale:    
Fair Value 3,733 3,146
Municipal securities    
Investments available for sale:    
Fair Value 2,388 2,187
Asset-backed securities    
Investments available for sale:    
Fair Value 744 683
Residential mortgage-backed securities    
Investments available for sale:    
Fair Value 471 444
Commercial mortgage- backed securities    
Investments available for sale:    
Fair Value 388 361
Interest rate swap agreements    
Liabilities    
Interest rate swap agreements 11 95
Level I    
Assets    
Cash and cash equivalents 12,123 5,342
Investments available for sale:    
Total assets at fair value 12,343 5,711
Liabilities    
Total liabilities at fair value 0 0
Level I | Interest rate swap agreements    
Liabilities    
Interest rate swap agreements 0 0
Level II    
Assets    
Cash and cash equivalents 0 0
Investments available for sale:    
Total assets at fair value 8,216 7,254
Liabilities    
Total liabilities at fair value 11 95
Level II | Interest rate swap agreements    
Liabilities    
Interest rate swap agreements 11 95
Level III    
Assets    
Cash and cash equivalents 0 0
Investments available for sale:    
Total assets at fair value 0 0
Liabilities    
Total liabilities at fair value 0 0
Level III | Interest rate swap agreements    
Liabilities    
Interest rate swap agreements 0 0
Investments available for sale    
Investments available for sale:    
Fair Value 7,768 7,068
Investments available for sale | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments available for sale:    
Fair Value 73 247
Investments available for sale | Corporate securities    
Investments available for sale:    
Fair Value 3,713 3,146
Investments available for sale | Municipal securities    
Investments available for sale:    
Fair Value 2,379 2,187
Investments available for sale | Asset-backed securities    
Investments available for sale:    
Fair Value 744 683
Investments available for sale | Residential mortgage-backed securities    
Investments available for sale:    
Fair Value 471 444
Investments available for sale | Commercial mortgage- backed securities    
Investments available for sale:    
Fair Value 388 361
Investments available for sale | Level I    
Investments available for sale:    
Fair Value 73 247
Investments available for sale | Level I | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments available for sale:    
Fair Value 73 247
Investments available for sale | Level I | Corporate securities    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level I | Municipal securities    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level I | Asset-backed securities    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level I | Residential mortgage-backed securities    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level I | Commercial mortgage- backed securities    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level II    
Investments available for sale:    
Fair Value 7,695 6,821
Investments available for sale | Level II | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level II | Corporate securities    
Investments available for sale:    
Fair Value 3,713 3,146
Investments available for sale | Level II | Municipal securities    
Investments available for sale:    
Fair Value 2,379 2,187
Investments available for sale | Level II | Asset-backed securities    
Investments available for sale:    
Fair Value 744 683
Investments available for sale | Level II | Residential mortgage-backed securities    
Investments available for sale:    
Fair Value 471 444
Investments available for sale | Level II | Commercial mortgage- backed securities    
Investments available for sale:    
Fair Value 388 361
Investments available for sale | Level III    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level III | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level III | Corporate securities    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level III | Municipal securities    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level III | Asset-backed securities    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level III | Residential mortgage-backed securities    
Investments available for sale:    
Fair Value 0 0
Investments available for sale | Level III | Commercial mortgage- backed securities    
Investments available for sale:    
Fair Value 0 0
Restricted Deposits    
Investments available for sale:    
Fair Value 658 555
Restricted Deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments available for sale:    
Fair Value 139 114
Restricted Deposits | Corporate securities    
Investments available for sale:    
Fair Value 20  
Restricted Deposits | Municipal securities    
Investments available for sale:    
Fair Value 9  
Restricted Deposits | Cash and cash equivalents    
Investments available for sale:    
Fair Value 8 8
Restricted Deposits | Certificates of deposit    
Investments available for sale:    
Fair Value 482 433
Restricted Deposits | Level I    
Investments available for sale:    
Fair Value 147 122
Restricted Deposits | Level I | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments available for sale:    
Fair Value 139 114
Restricted Deposits | Level I | Corporate securities    
Investments available for sale:    
Fair Value 0  
Restricted Deposits | Level I | Municipal securities    
Investments available for sale:    
Fair Value 0  
Restricted Deposits | Level I | Cash and cash equivalents    
Investments available for sale:    
Fair Value 8 8
Restricted Deposits | Level I | Certificates of deposit    
Investments available for sale:    
Fair Value 0 0
Restricted Deposits | Level II    
Investments available for sale:    
Fair Value 511 433
Restricted Deposits | Level II | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments available for sale:    
Fair Value 0 0
Restricted Deposits | Level II | Corporate securities    
Investments available for sale:    
Fair Value 20  
Restricted Deposits | Level II | Municipal securities    
Investments available for sale:    
Fair Value 9  
Restricted Deposits | Level II | Cash and cash equivalents    
Investments available for sale:    
Fair Value 0 0
Restricted Deposits | Level II | Certificates of deposit    
Investments available for sale:    
Fair Value 482 433
Restricted Deposits | Level III    
Investments available for sale:    
Fair Value 0 0
Restricted Deposits | Level III | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments available for sale:    
Fair Value 0 0
Restricted Deposits | Level III | Corporate securities    
Investments available for sale:    
Fair Value 0  
Restricted Deposits | Level III | Municipal securities    
Investments available for sale:    
Fair Value 0  
Restricted Deposits | Level III | Cash and cash equivalents    
Investments available for sale:    
Fair Value 0 0
Restricted Deposits | Level III | Certificates of deposit    
Investments available for sale:    
Fair Value 0 $ 0
Total other long-term assets    
Investments available for sale:    
Derivative asset 10  
Total other long-term assets | Interest rate swap agreements    
Investments available for sale:    
Derivative asset 10  
Total other long-term assets | Level I    
Investments available for sale:    
Derivative asset 0  
Total other long-term assets | Level I | Interest rate swap agreements    
Investments available for sale:    
Derivative asset 0  
Total other long-term assets | Level II    
Investments available for sale:    
Derivative asset 10  
Total other long-term assets | Level II | Interest rate swap agreements    
Investments available for sale:    
Derivative asset 10  
Total other long-term assets | Level III    
Investments available for sale:    
Derivative asset 0  
Total other long-term assets | Level III | Interest rate swap agreements    
Investments available for sale:    
Derivative asset $ 0