Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits - Beginning of year	$ 55	$ 60	$ 56
Increases for tax positions taken during the current year	10	9	9
Increases for tax positions taken in prior years	0	10	6
Decreases for tax positions taken in prior years	(10)	(21)	(7)
Decreases for settlements	(1)	(1)	(2)
Lapse of the statute of limitations	0	(2)	(2)
Unrecognized tax benefits - End of year	$ 54	$ 55	$ 60