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, 2016
Dec. 31, 2015
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost $ 5,040 $ 2,225
Gross Unrealized Gains 76 5
Gross Unrealized Losses (5) (12)
Fair Value 5,111 2,218
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 288 431
Gross Unrealized Gains 2 0
Gross Unrealized Losses 0 (2)
Fair Value 290 429
Corporate securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 1,915 859
Gross Unrealized Gains 30 2
Gross Unrealized Losses (2) (8)
Fair Value 1,943 853
Restricted certificates of deposit    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 6 5
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 6 5
Restricted cash equivalents    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 70 78
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 70 78
Municipal securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 1,638 496
Gross Unrealized Gains 24 2
Gross Unrealized Losses (1) (1)
Fair Value 1,661 497
Asset-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 289 163
Gross Unrealized Gains 2 0
Gross Unrealized Losses 0 (1)
Fair Value 291 162
Residential mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 215 66
Gross Unrealized Gains 3 1
Gross Unrealized Losses (1) 0
Fair Value 217 67
Commercial mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 341 40
Gross Unrealized Gains 15 0
Gross Unrealized Losses (1) 0
Fair Value 355 40
Cost and equity method investments    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 162 71
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 162 71
Life insurance contracts    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 116 16
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value $ 116 $ 16