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
Mar. 31, 2019
Dec. 31, 2018
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost $ 8,411 $ 8,208
Gross Unrealized Gains 85 21
Gross Unrealized Losses (31) (91)
Fair Value 8,465 8,138
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 251 362
Gross Unrealized Gains 1 1
Gross Unrealized Losses (1) (2)
Fair Value 251 361
Corporate securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 3,318 3,190
Gross Unrealized Gains 40 8
Gross Unrealized Losses (16) (52)
Fair Value 3,342 3,146
Restricted certificates of deposit    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 474 433
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 474 433
Restricted cash equivalents    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 6 8
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 6 8
Municipal securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 2,260 2,196
Gross Unrealized Gains 33 9
Gross Unrealized Losses (3) (18)
Fair Value 2,290 2,187
Asset-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 728 686
Gross Unrealized Gains 3 1
Gross Unrealized Losses (3) (4)
Fair Value 728 683
Residential mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 457 452
Gross Unrealized Gains 4 1
Gross Unrealized Losses (6) (9)
Fair Value 455 444
Commercial mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 378 366
Gross Unrealized Gains 4 1
Gross Unrealized Losses (2) (6)
Fair Value 380 361
Private equity investments    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 402 387
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 402 387
Life insurance contracts    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 137 128
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value $ 137 $ 128