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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

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