Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Valuation Allowance [Line Items]
AMT credit carryforward	$ 379	$ 357
Operating loss carryforwards	$ 9,500
Operating loss carryforwards, expiration dates	Jan. 01, 2024
Valuation allowance	$ 56	46	$ 177	$ 10,963
Income before income taxes	7,157	1,072	2,527
Income tax benefit related to tax allocation	1,900
Deferred income taxes	1,900
Income tax expense related to tax allocation	321
Unrecognized tax benefits	32	40	37
Release of valuation allowance
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	$ (10)	$ (28)	$ (8,310)