Asset Impairment Expense (Impairment of Long-Lived Assets Held and Used by Asset) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Asset impairment expense		$ 285	$ 91	$ 95
Kilroot [Member]
Debt Instrument [Line Items]
Asset impairment expense	$ 121	121	0	0
buffalo gap III [Member]
Debt Instrument [Line Items]
Asset impairment expense	116	116	0	0
UK Wind Development Projects [Domain]
Debt Instrument [Line Items]
Asset impairment expense	$ 37	37
Ebute [Member]
Debt Instrument [Line Items]
Asset impairment expense		0	67	0
UK Wind (Newfield) [Member]
Debt Instrument [Line Items]
Asset impairment expense			12	0
DP&L (East Bend)
Debt Instrument [Line Items]
Asset impairment expense		0	12	0
Beaver Valley
Debt Instrument [Line Items]
Asset impairment expense		0	0	46
Conesville (DP&L)
Debt Instrument [Line Items]
Asset impairment expense		0	0	26
Itabo (San Lorenzo)
Debt Instrument [Line Items]
Asset impairment expense		0	0	16
Other
Debt Instrument [Line Items]
Asset impairment expense		$ 11	$ 0	$ 7