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, 2018
Dec. 31, 2017
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost $ 7,518 $ 5,982
Gross Unrealized Gains 6 27
Gross Unrealized Losses (108) (31)
Fair Value 7,416 5,978
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 259 311
Gross Unrealized Gains 0 0
Gross Unrealized Losses (3) (2)
Fair Value 256 309
Corporate securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 2,774 2,208
Gross Unrealized Gains 4 12
Gross Unrealized Losses (44) (10)
Fair Value 2,734 2,210
Restricted certificates of deposit    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 430 4
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 430 4
Restricted cash equivalents    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 7 17
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 7 17
Municipal securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 2,231 2,085
Gross Unrealized Gains 2 12
Gross Unrealized Losses (35) (10)
Fair Value 2,198 2,087
Asset-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 598 437
Gross Unrealized Gains 0 1
Gross Unrealized Losses (4) (1)
Fair Value 594 437
Residential mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 382 337
Gross Unrealized Gains 0 1
Gross Unrealized Losses (14) (6)
Fair Value 368 332
Commercial mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 317 272
Gross Unrealized Gains 0 1
Gross Unrealized Losses (8) (2)
Fair Value 309 271
Private equity investments    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 382 176
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 382 176
Life insurance contracts    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 138 135
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value $ 138 $ 135