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Short-term and Long-term Investments, Restricted Deposits - Schedule by investment Type (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Investments And Restricted Deposits By Type [Line Items]      
Amortized Cost $ 8,208 $ 5,982  
Gross Unrealized Gains 21 27  
Gross Unrealized Losses (91) (31)  
Fair Value 8,138   $ 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 362 311  
Gross Unrealized Gains 1 0  
Gross Unrealized Losses (2) (2)  
Fair Value 361 309  
Corporate securities      
Schedule Of Investments And Restricted Deposits By Type [Line Items]      
Amortized Cost 3,190 2,208  
Gross Unrealized Gains 8 12  
Gross Unrealized Losses (52) (10)  
Fair Value 3,146 2,210  
Restricted certificates of deposit      
Schedule Of Investments And Restricted Deposits By Type [Line Items]      
Amortized Cost 433 4  
Gross Unrealized Gains 0 0  
Gross Unrealized Losses 0 0  
Fair Value 433 4  
Restricted cash equivalents      
Schedule Of Investments And Restricted Deposits By Type [Line Items]      
Amortized Cost 8 17  
Gross Unrealized Gains 0 0  
Gross Unrealized Losses 0 0  
Fair Value 8 17  
Municipal securities      
Schedule Of Investments And Restricted Deposits By Type [Line Items]      
Amortized Cost 2,196 2,085  
Gross Unrealized Gains 9 12  
Gross Unrealized Losses (18) (10)  
Fair Value 2,187 2,087  
Asset-backed securities      
Schedule Of Investments And Restricted Deposits By Type [Line Items]      
Amortized Cost 686 437  
Gross Unrealized Gains 1 1  
Gross Unrealized Losses (4) (1)  
Fair Value 683 437  
Residential mortgage-backed securities      
Schedule Of Investments And Restricted Deposits By Type [Line Items]      
Amortized Cost 452 337  
Gross Unrealized Gains 1 1  
Gross Unrealized Losses (9) (6)  
Fair Value 444 332  
Commercial mortgage- backed securities      
Schedule Of Investments And Restricted Deposits By Type [Line Items]      
Amortized Cost 366 272  
Gross Unrealized Gains 1 1  
Gross Unrealized Losses (6) (2)  
Fair Value 361 271  
Private equity investments      
Schedule Of Investments And Restricted Deposits By Type [Line Items]      
Amortized Cost 387 176  
Gross Unrealized Gains 0 0  
Gross Unrealized Losses 0 0  
Fair Value 387 176  
Life insurance contracts      
Schedule Of Investments And Restricted Deposits By Type [Line Items]      
Amortized Cost 128 135  
Gross Unrealized Gains 0 0  
Gross Unrealized Losses 0 0  
Fair Value $ 128 $ 135