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, 2018
Dec. 31, 2017
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost $ 6,256 $ 5,982
Gross Unrealized Gains 9 27
Gross Unrealized Losses (83) (31)
Fair Value 6,182 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 260 311
Gross Unrealized Gains 0 0
Gross Unrealized Losses (3) (2)
Fair Value 257 309
Corporate securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 2,242 2,208
Gross Unrealized Gains 5 12
Gross Unrealized Losses (33) (10)
Fair Value 2,214 2,210
Restricted certificates of deposit    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 4 4
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 4 4
Restricted cash equivalents    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 16 17
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 16 17
Municipal securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 2,187 2,085
Gross Unrealized Gains 3 12
Gross Unrealized Losses (28) (10)
Fair Value 2,162 2,087
Asset-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 492 437
Gross Unrealized Gains 1 1
Gross Unrealized Losses (3) (1)
Fair Value 490 437
Residential mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 345 337
Gross Unrealized Gains 0 1
Gross Unrealized Losses (10) (6)
Fair Value 335 332
Commercial mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 287 272
Gross Unrealized Gains 0 1
Gross Unrealized Losses (6) (2)
Fair Value 281 271
Fair value and equity method investments    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 290 176
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 290 176
Life insurance contracts    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Amortized Cost 133 135
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value $ 133 $ 135