Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of tnrecognized tax benefits
Balance at beginning of year	$ 86	$ 93	$ 111
Additions for tax positions of prior years	4	1	3
Reductions for tax positions of prior years	(2)	(3)	(3)
Settlements	(60)	(1)	(18)
Reductions for lapse of statute of limitations	(8)	(4)	0
Balance at end of year	$ 20	$ 86	$ 93