Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Schedule of Reconciliation of Level 3 Investments Securities (Footnotes) (Details)

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Fair Value Measurements - Schedule of Reconciliation of Level 3 Investments Securities (Footnotes) (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Estimated Fair Value    
Securities with a loss shorter than 12 months $ 103 $ 26
Securities with a loss shorter than 12 months 2 1
Unrealized Losses    
Securities with a loss longer than 12 months 44 46
Securities with a loss longer than 12 months $ 3 $ 2