Asset Impairment Expense (Details)	3 Months Ended					9 Months Ended
	Sep. 30, 2015 USD ($) MW	Jun. 30, 2015 USD ($) project	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($) MW	Sep. 30, 2015 USD ($) MW	Sep. 30, 2014 USD ($)
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use	$ 231,000,000		$ 15,000,000			$ 276,000,000	$ 90,000,000
Kilroot [Member]
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use	113,000,000		0			113,000,000	0
Assets, fair value	78,000,000					78,000,000
buffalo gap III [Member]
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use	$ 118,000,000		0			$ 118,000,000	0
Generation Capacity Mega Watts | MW	170					170
Assets, fair value	$ 116,000,000					$ 116,000,000
UK Wind Development Projects [Domain]
Asset Impairment Expense [Line Items]
number of projects no longer being pursued | project		2
number of impaired projects | project		4
Long-Lived Assets		$ 38,000,000
Impairment of Long-Lived Assets Held-for-use	0					37,000,000
Assets, fair value		$ 1,000,000
East Bend DPL [Member]
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use	0		0		$ 12,000,000	0	12,000,000
Generation Capacity Mega Watts | MW					186
Assets, fair value					$ 2,000,000
Ebute [Member]
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use	0		15,000,000	$ 52,000,000		0	67,000,000
Assets, fair value			36,000,000	47,000,000			36,000,000
UK Wind (Newfield) [Member]
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use			0	$ 11,000,000			11,000,000
Assets, fair value			0				0
Other Impairment [Member]
Asset Impairment Expense [Line Items]
Impairment of Long-Lived Assets Held-for-use	$ 0		$ 0			$ 8,000,000	$ 0