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, 2018
Dec. 31, 2017
Investments    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, one year or less $ 517 $ 474
Amortized cost, one year through five years 2,674 2,424
Amortized cost, five years through ten years 2,277 1,773
Amortized cost, greater than ten years 202 129
Amortized cost, asset-backed securities 1,297 1,046
Amortized cost, total 6,967 5,846
Fair value, one year or less 515 474
Fair value, one year through five years 2,636 2,420
Fair value, five years through ten years 2,242 1,779
Fair value, greater than ten years 202 130
Fair value, asset-backed securities 1,271 1,040
Fair value, total 6,866 5,843
Restricted Deposits    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, one year or less 201 48
Amortized cost, one year through five years 350 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 551 136
Fair value, one year or less 201 47
Fair value, one year through five years 349 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 $ 550 $ 135