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, 2017
Dec. 31, 2016
Assets    
Cash and cash equivalents $ 4,281 $ 3,930
U.S. Treasury securities and obligations of U.S. government corporations and agencies 201 236
Corporate securities 2,110 1,932
Municipal securities 1,989 1,733
Asset-backed securities 378 317
Total investments 5,220 4,771
Cash and cash equivalents 6 6
Certificates of deposit 4 5
U.S. Treasury securities and obligations of U.S. government corporations and agencies 128 127
Total restricted deposits 138 138
Interest rate swap agreements 4 4
Total assets at fair value 9,643 8,843
Liabilities    
Interest rate swap agreements 53 62
Total liabilities at fair value 53 62
Residential mortgage-backed securities    
Assets    
Mortgage-backed securities 306 215
Commercial mortgage-backed securities    
Assets    
Mortgage-backed securities 236 338
Level I    
Assets    
Cash and cash equivalents 4,281 3,930
U.S. Treasury securities and obligations of U.S. government corporations and agencies 201 221
Corporate securities 0 0
Municipal securities 0 0
Asset-backed securities 0 0
Total investments 201 221
Cash and cash equivalents 6 6
Certificates of deposit 4 5
U.S. Treasury securities and obligations of U.S. government corporations and agencies 128 127
Total restricted deposits 138 138
Interest rate swap agreements 0 0
Total assets at fair value 4,620 4,289
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 15
Corporate securities 2,110 1,932
Municipal securities 1,989 1,733
Asset-backed securities 378 317
Total investments 5,019 4,550
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 4 4
Total assets at fair value 5,023 4,554
Liabilities    
Interest rate swap agreements 53 62
Total liabilities at fair value 53 62
Level II | Residential mortgage-backed securities    
Assets    
Mortgage-backed securities 306 215
Level II | Commercial mortgage-backed securities    
Assets    
Mortgage-backed securities 236 338
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