Quarterly report pursuant to Section 13 or 15(d)

Short-term and Long-term Investments, Restricted Deposits (Contractual Maturities of Short-Term and Long-Term Investments and Restricted Deposits) (Details)

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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
Sep. 30, 2017
Dec. 31, 2016
Investments    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost, one year or less $ 533 $ 500
Amortized cost, one year through five years 2,149 1,982
Amortized cost, five years through ten years 1,680 1,101
Amortized cost, greater than ten years 219 633
Amortized cost, asset-backed securities 923 879
Amortized cost, total 5,504 5,095
Fair value, one year or less 533 500
Fair value, one year through five years 2,162 1,974
Fair value, five years through ten years 1,687 1,089
Fair value, greater than ten years 220 617
Fair value, asset-backed securities 920 870
Fair value, total 5,522 5,050
Restricted Deposits    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost, one year or less 65 91
Amortized cost, one year through five years 73 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 138 138
Fair value, one year or less 65 91
Fair value, one year through five years 73 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 $ 138 $ 138