v3.8.0.1
Short-term and Long-term Investments, Restricted Deposits - Contractual Maturities of Short-Term And Long-Term Investments And Restricted Deposits (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Investments    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost, One year or less $ 474 $ 500
Amortized Cost, One year through five years 2,424 1,982
Amortized Cost, Five years through ten years 1,773 1,101
Amortized Cost, Greater than ten years 129 633
Amortized Cost, Asset-backed securities 1,046 879
Amortized Cost, Total 5,846 5,095
Investments And Restricted Deposits Contractual Maturities Within One Year Fair Value 474 500
Fair Value, One year through five years 2,420 1,974
Fair Value, Five years through ten years 1,779 1,089
Fair Value, Greater than ten years 130 617
Fair Value, Asset-backed securities 1,040 870
Fair Value, Total 5,843 5,050
Restricted Deposits    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost, One year or less 48 91
Amortized Cost, One year through five years 88 47
Amortized Cost, Five years through ten years 0 0
Amortized Cost, Greater than ten years 0 0
Amortized Cost, Asset-backed securities 0 0
Amortized Cost, Total 136 138
Investments And Restricted Deposits Contractual Maturities Within One Year Fair Value 47 91
Fair Value, One year through five years 88 47
Fair Value, Five years through ten years 0 0
Fair Value, Greater than ten years 0 0
Fair Value, Asset-backed securities 0 0
Fair Value, Total $ 135 $ 138