Quarterly report pursuant to Section 13 or 15(d)

Short-term and Long-term Investments, Restricted Deposits (By Investment Type) (Details)

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Short-term and Long-term Investments, Restricted Deposits (By Investment Type) (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost $ 5,642 $ 5,233
Gross Unrealized Gains 38 15
Gross Unrealized Losses (20) (60)
Fair Value 5,660 5,188
U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 330 364
Gross Unrealized Gains 0 0
Gross Unrealized Losses (1) (1)
Fair Value 329 363
Corporate securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 2,098 1,933
Gross Unrealized Gains 18 12
Gross Unrealized Losses (6) (13)
Fair Value 2,110 1,932
Restricted certificates of deposit    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 4 5
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 4 5
Restricted cash equivalents    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 6 6
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 6 6
Municipal securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 1,979 1,767
Gross Unrealized Gains 16 1
Gross Unrealized Losses (6) (35)
Fair Value 1,989 1,733
Asset-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 376 317
Gross Unrealized Gains 2 1
Gross Unrealized Losses 0 (1)
Fair Value 378 317
Residential mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 309 219
Gross Unrealized Gains 1 1
Gross Unrealized Losses (4) (5)
Fair Value 306 215
Commercial mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 238 343
Gross Unrealized Gains 1 0
Gross Unrealized Losses (3) (5)
Fair Value 236 338
Cost and equity method investments    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 171 163
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 171 163
Life insurance contracts    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 131 116
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value $ 131 $ 116