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
Jun. 30, 2017
Dec. 31, 2016
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost $ 5,531 $ 5,233
Gross Unrealized Gains 31 15
Gross Unrealized Losses (23) (60)
Fair Value 5,539 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 329 364
Gross Unrealized Gains 0 0
Gross Unrealized Losses (1) (1)
Fair Value 328 363
Corporate securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 2,048 1,933
Gross Unrealized Gains 17 12
Gross Unrealized Losses (6) (13)
Fair Value 2,059 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 8 6
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 8 6
Municipal securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 1,923 1,767
Gross Unrealized Gains 11 1
Gross Unrealized Losses (8) (35)
Fair Value 1,926 1,733
Asset-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 373 317
Gross Unrealized Gains 1 1
Gross Unrealized Losses 0 (1)
Fair Value 374 317
Residential mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 231 219
Gross Unrealized Gains 1 1
Gross Unrealized Losses (4) (5)
Fair Value 228 215
Commercial mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 322 343
Gross Unrealized Gains 1 0
Gross Unrealized Losses (4) (5)
Fair Value 319 338
Cost and equity method investments    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 167 163
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 167 163
Life insurance contracts    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 126 116
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value $ 126 $ 116