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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, 2018
Dec. 31, 2017
Assets    
Cash and cash equivalents $ 5,342 $ 4,072
Investments available for sale:    
U.S. Treasury securities and obligations of U.S. government corporations and agencies 247 195
Corporate securities 3,146 2,210
Municipal securities 2,187 2,087
Asset-backed securities 683 437
Total investments 7,068 5,532
Restricted deposits available for sale:    
Cash and cash equivalents 8 17
Certificates of deposit 433 4
U.S. Treasury securities and obligations of U.S. government corporations and agencies 114 114
Total restricted deposits 555 135
Other long-term assets:    
Interest rate swap agreements   1
Total assets at fair value 12,965 9,740
Liabilities    
Interest rate swap agreements 95 72
Total liabilities at fair value 95 72
Residential mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 444 332
Commercial mortgage- backed securities    
Investments available for sale:    
Mortgage-backed securities 361 271
Level I    
Assets    
Cash and cash equivalents 5,342 4,072
Investments available for sale:    
U.S. Treasury securities and obligations of U.S. government corporations and agencies 247 195
Corporate securities 0 0
Municipal securities 0 0
Asset-backed securities 0 0
Total investments 247 195
Restricted deposits available for sale:    
Cash and cash equivalents 8 17
Certificates of deposit 0 4
U.S. Treasury securities and obligations of U.S. government corporations and agencies 114 114
Total restricted deposits 122 135
Other long-term assets:    
Interest rate swap agreements   0
Total assets at fair value 5,711 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 3,146 2,210
Municipal securities 2,187 2,087
Asset-backed securities 683 437
Total investments 6,821 5,337
Restricted deposits available for sale:    
Cash and cash equivalents 0 0
Certificates of deposit 433 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Total restricted deposits 433 0
Other long-term assets:    
Interest rate swap agreements   1
Total assets at fair value 7,254 5,338
Liabilities    
Interest rate swap agreements 95 72
Total liabilities at fair value 95 72
Level II | Residential mortgage-backed securities    
Investments available for sale:    
Mortgage-backed securities 444 332
Level II | Commercial mortgage- backed securities    
Investments available for sale:    
Mortgage-backed securities 361 271
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