Components of Income Tax Expense from Continuing Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
State	$ 20	$ 10	$ 11
Federal	1		15
Total current income taxes	21	10	26
State	24	32	31
Federal	258	265	224
Amortization of deferred investment tax credits	(1)	(1)	(1)
Total deferred income taxes	281	296	254
Provision for income taxes	$ 302	$ 306	$ 280