Income Taxes - Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income taxes related to adjustments included in common shareholder’s equity:
Provision (benefit) for income taxes	$ 64	$ (203)	$ (63)	$ (56)	$ (114)	$ (129)	$ (200)	$ (28)	$ (258)	$ (471)	$ 224
Unrealized gains on cash flow hedging instruments									9	21	24
Changes in funded status of postretirement benefit plans									(2)	25	(2)
Total income taxes									(251)	(425)	246
Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current provision (benefit)									(278)	56	108
Deferred income tax provision (benefit)									20	(527)	116
Total income tax expense (benefit)	$ 64	$ (203)	$ (63)	$ (56)	$ (114)	$ (129)	$ (200)	$ (28)	(258)	(471)	224
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred income tax provision excluding the effects of other components									895	28	473
Increase in Federal operating loss carryforwards									(859)	(479)	(366)
Decrease in State valuation allowances									(17)	(171)	(53)
Increase in State operating loss carryforwards and other temporary differences									1	95	42
Decrease in foreign and general business tax credits									0	0	20
Deferred income tax provision (benefit)									$ 20	$ (527)	$ 116