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, 2018
Dec. 31, 2017
Assets    
Cash and cash equivalents $ 5,668 $ 4,072
U.S. Treasury securities and obligations of U.S. government corporations and agencies 137 195
Corporate securities 2,214 2,210
Municipal securities 2,162 2,087
Asset-backed securities 490 437
Total investments 5,619 5,532
Cash and cash equivalents 16 17
Certificates of deposit 4 4
U.S. Treasury securities and obligations of U.S. government corporations and agencies 120 114
Total restricted deposits 140 135
Interest rate swap agreements 0 1
Total assets at fair value 11,427 9,740
Liabilities    
Interest rate swap agreements 120 72
Total liabilities at fair value 120 72
Residential mortgage-backed securities    
Assets    
Mortgage-backed securities 335 332
Commercial mortgage-backed securities    
Assets    
Mortgage-backed securities 281 271
Level I    
Assets    
Cash and cash equivalents 5,668 4,072
U.S. Treasury securities and obligations of U.S. government corporations and agencies 137 195
Corporate securities 0 0
Municipal securities 0 0
Asset-backed securities 0 0
Total investments 137 195
Cash and cash equivalents 16 17
Certificates of deposit 4 4
U.S. Treasury securities and obligations of U.S. government corporations and agencies 120 114
Total restricted deposits 140 135
Interest rate swap agreements 0 0
Total assets at fair value 5,945 4,402
Liabilities    
Interest rate swap agreements 0 0
Total liabilities at fair value 0 0
Level I | Residential mortgage-backed securities    
Assets    
Mortgage-backed securities 0 0
Level I | Commercial mortgage-backed securities    
Assets    
Mortgage-backed securities 0 0
Level II    
Assets    
Cash and cash equivalents 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Corporate securities 2,214 2,210
Municipal securities 2,162 2,087
Asset-backed securities 490 437
Total investments 5,482 5,337
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
Interest rate swap agreements 0 1
Total assets at fair value 5,482 5,338
Liabilities    
Interest rate swap agreements 120 72
Total liabilities at fair value 120 72
Level II | Residential mortgage-backed securities    
Assets    
Mortgage-backed securities 335 332
Level II | Commercial mortgage-backed securities    
Assets    
Mortgage-backed securities 281 271
Level III    
Assets    
Cash and cash equivalents 0 0
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
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
Interest rate swap agreements 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    
Assets    
Mortgage-backed securities 0 0
Level III | Commercial mortgage-backed securities    
Assets    
Mortgage-backed securities $ 0 $ 0