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, 2019
Dec. 31, 2018
Investments    
Investments, Amortized Cost    
One year or less $ 610 $ 647
One year through five years 3,112 3,026
Five years through ten years 2,477 2,387
Greater than ten years 67 88
Asset-backed securities 1,563 1,504
Total Investments, Amortized Cost 7,829 7,652
Investments, Fair Value    
One year or less 609 646
One year through five years 3,130 2,998
Five years through ten years 2,512 2,362
Greater than ten years 69 89
Asset-backed securities 1,563 1,488
Total Investments, Fair Value 7,883 7,583
Restricted Deposits    
Restricted Deposits, Amortized Cost    
One year or less 515 205
One year through five years 67 351
Five years through ten years 0 0
Greater than ten years 0 0
Asset-backed securities 0 0
Total Restricted Deposits, Amortized Cost 582 556
Restricted Deposits, Fair Value    
One year or less 515 205
One year through five years 67 350
Five years through ten years 0 0
Greater than ten years 0 0
Asset-backed securities 0 0
Total Restricted Deposits, Fair Value $ 582 $ 555