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
Mar. 31, 2018
Dec. 31, 2017
Investments    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost, one year or less $ 432 $ 474
Amortized cost, one year through five years 2,439 2,424
Amortized cost, five years through ten years 1,920 1,773
Amortized cost, greater than ten years 200 129
Amortized cost, asset-backed securities 1,124 1,046
Amortized cost, total 6,115 5,846
Fair value, one year or less 430 474
Fair value, one year through five years 2,411 2,420
Fair value, five years through ten years 1,894 1,779
Fair value, greater than ten years 201 130
Fair value, asset-backed securities 1,106 1,040
Fair value, total 6,042 5,843
Restricted Deposits    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost, one year or less 45 48
Amortized cost, one year through five years 96 88
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 141 136
Fair value, one year or less 45 47
Fair value, one year through five years 95 88
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 $ 140 $ 135