Income Taxes Income Taxes - Effective Income Tax Expense (Benefit) Table (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	35.00%	35.00%	35.00%
Income tax expense (benefit) at the federal statutory rate (35%)	$ 333	$ (1,606)	$ (6,684)
State income taxes (net of federal income tax benefit)	66	(30)	(406)
Remeasurement of deferred tax assets and liabilities	1,266	0	0
Change in valuation allowance	(1,676)	1,423	2,727
Other	13	23	(100)
Deferred income taxes	$ 2	$ (190)	$ (4,463)