Quarterly report pursuant to Section 13 or 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
Mar. 31, 2019
Dec. 31, 2018
Assets    
Cash and cash equivalents $ 6,345 $ 5,342
Investments available for sale:    
U.S. Treasury securities and obligations of U.S. government corporations and agencies 149 247
Corporate securities 3,342 3,146
Municipal securities 2,290 2,187
Asset-backed securities 728 683
Total investments 7,344 7,068
Restricted deposits available for sale:    
Cash and cash equivalents 6 8
Certificates of deposit 474 433
U.S. Treasury securities and obligations of U.S. government corporations and agencies 102 114
Total restricted deposits 582 555
Total assets at fair value 14,271 12,965
Liabilities    
Interest rate swap agreements 61 95
Total liabilities at fair value 61 95
Residential mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 455 444
Commercial mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 380 361
Level I    
Assets    
Cash and cash equivalents 6,345 5,342
Investments available for sale:    
U.S. Treasury securities and obligations of U.S. government corporations and agencies 149 247
Corporate securities 0 0
Municipal securities 0 0
Asset-backed securities 0 0
Total investments 149 247
Restricted deposits available for sale:    
Cash and cash equivalents 6 8
Certificates of deposit 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 102 114
Total restricted deposits 108 122
Total assets at fair value 6,602 5,711
Liabilities    
Interest rate swap agreements 0 0
Total liabilities at fair value 0 0
Level I | Residential mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 0 0
Level I | Commercial mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 0 0
Level II    
Assets    
Cash and cash equivalents 0 0
Investments available for sale:    
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Corporate securities 3,342 3,146
Municipal securities 2,290 2,187
Asset-backed securities 728 683
Total investments 7,195 6,821
Restricted deposits available for sale:    
Cash and cash equivalents 0 0
Certificates of deposit 474 433
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Total restricted deposits 474 433
Total assets at fair value 7,669 7,254
Liabilities    
Interest rate swap agreements 61 95
Total liabilities at fair value 61 95
Level II | Residential mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 455 444
Level II | Commercial mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 380 361
Level III    
Assets    
Cash and cash equivalents 0 0
Investments available for sale:    
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Corporate securities 0 0
Municipal securities 0 0
Asset-backed securities 0 0
Total investments 0 0
Restricted deposits available for sale:    
Cash and cash equivalents 0 0
Certificates of deposit 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Total restricted deposits 0 0
Total assets at fair value 0 0
Liabilities    
Interest rate swap agreements 0 0
Total liabilities at fair value 0 0
Level III | Residential mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 0 0
Level III | Commercial mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities $ 0 $ 0