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
Sep. 30, 2018
Dec. 31, 2017
Assets    
Cash and cash equivalents $ 6,847 $ 4,072
Investments available for sale:    
U.S. Treasury securities and obligations of U.S. government corporations and agencies 143 195
Corporate securities 2,734 2,210
Municipal securities 2,198 2,087
Asset-backed securities 594 437
Total investments 6,346 5,532
Restricted deposits available for sale:    
Cash and cash equivalents 7 17
Certificates of deposit 430 4
U.S. Treasury securities and obligations of U.S. government corporations and agencies 113 114
Total restricted deposits 550 135
Other long-term assets:    
Interest rate swap agreements   1
Total assets at fair value 13,743 9,740
Liabilities    
Interest rate swap agreements 147 72
Total liabilities at fair value 147 72
Residential mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 368 332
Commercial mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 309 271
Level I    
Assets    
Cash and cash equivalents 6,847 4,072
Investments available for sale:    
U.S. Treasury securities and obligations of U.S. government corporations and agencies 143 195
Corporate securities 0 0
Municipal securities 0 0
Asset-backed securities 0 0
Total investments 143 195
Restricted deposits available for sale:    
Cash and cash equivalents 7 17
Certificates of deposit 430 4
U.S. Treasury securities and obligations of U.S. government corporations and agencies 113 114
Total restricted deposits 550 135
Other long-term assets:    
Interest rate swap agreements   0
Total assets at fair value 7,540 4,402
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 2,734 2,210
Municipal securities 2,198 2,087
Asset-backed securities 594 437
Total investments 6,203 5,337
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
Other long-term assets:    
Interest rate swap agreements   1
Total assets at fair value 6,203 5,338
Liabilities    
Interest rate swap agreements 147 72
Total liabilities at fair value 147 72
Level II | Residential mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 368 332
Level II | Commercial mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 309 271
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
Other long-term assets:    
Interest rate swap agreements   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