Income Taxes (Reconciliation of Provision for Income Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Abstract]
Expected provision (benefit) at federal statutory rate	$ 333	$ (935)	$ (2,296)
Increase (decrease) resulting from:
State income taxes, net of federal income tax effect	16	(79)	(194)
Nondeductible expenses
Rate impacts due to tax reform	370
Changes to valuation allowance due to tax reform	(370)
AMT tax reform impact - valuation allowance release	(68)
Change in uncertain tax positions	(5)	(19)	(7)
Change in valuation allowance	(364)	1,002	495
Other	(5)	2	(3)
Benefit for Income Taxes	$ (93)	$ (29)	$ (2,005)