Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of tnrecognized tax benefits
Balance at beginning of year	$ 93	$ 111	$ 165
Additions for tax positions of prior years	1	3	3
Reductions for tax positions of prior years	(3)	(3)	(41)
Settlements	(1)	(18)	(8)
Reductions for lapse of statute of limitations	(4)	0	(8)
Balance at end of year	$ 86	$ 93	$ 111