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, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 2,982 $ 1,760
U.S. Treasury securities and obligations of U.S. government corporations and agencies 229 397
Corporate securities 1,943 853
Municipal securities 1,661 497
Asset-backed securities 291 162
Total investments 4,696 2,016
Cash and cash equivalents 70 78
Certificates of deposit 6 5
U.S. Treasury securities and obligations of U.S. government corporations and agencies 61 32
Total restricted deposits 137 115
Interest rate swap agreements 25 11
Total assets at fair value 7,840 3,902
Interest rate swap agreements   2
Total liabilities at fair value   2
Residential mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Mortgage-backed securities 217 67
Commercial mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Mortgage-backed securities 355 40
Level I    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 2,982 1,760
U.S. Treasury securities and obligations of U.S. government corporations and agencies 215 325
Corporate securities 0 0
Municipal securities 0 0
Asset-backed securities 0 0
Total investments 215 325
Cash and cash equivalents 70 78
Certificates of deposit 6 5
U.S. Treasury securities and obligations of U.S. government corporations and agencies 61 32
Total restricted deposits 137 115
Interest rate swap agreements 0 0
Total assets at fair value 3,334 2,200
Interest rate swap agreements   0
Total liabilities at fair value   0
Level I | Residential mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Mortgage-backed securities 0 0
Level I | Commercial mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Mortgage-backed securities 0 0
Level II    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 14 72
Corporate securities 1,943 853
Municipal securities 1,661 497
Asset-backed securities 291 162
Total investments 4,481 1,691
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 25 11
Total assets at fair value 4,506 1,702
Interest rate swap agreements   2
Total liabilities at fair value   2
Level II | Residential mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Mortgage-backed securities 217 67
Level II | Commercial mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Mortgage-backed securities 355 40
Level III    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
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
Interest rate swap agreements   0
Total liabilities at fair value   0
Level III | Residential mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Mortgage-backed securities 0 0
Level III | Commercial mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Mortgage-backed securities $ 0 $ 0