Income Taxes - Reconciliation of the Beginning and Ending Amount of Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Balance, beginning of year	$ 89	$ 67	$ 36
Increases to tax positions related to the current year	16	15	13
Increases to tax positions related to the prior year	1	7	18
Reductions due to lapsed statute of limitations	(1)
Balance, end of year	$ 105	$ 89	$ 67