Income Taxes - Reconciliation of Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance, beginning of period	$ 117	$ 87	$ 109
Decrease related to expiration of statute of limitations	(3)	(5)	(6)
Decrease related to prior years tax positions	(25)	(1)	(9)
Increase related to prior years tax positions	15	28	1
Increase related to current year tax positions	15	8	8
Decrease related to settlements			(16)
Balance, end of period	$ 119	$ 117	$ 87