Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Summary of Fair Value and Carrying Value of Financial Assets and Liabilities (Footnotes) (Details)

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Fair Value Measurements - Summary of Fair Value and Carrying Value of Financial Assets and Liabilities (Footnotes) (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Fair Value Disclosures [Abstract]    
Amortized cost of investments $ 169 $ 153