Goodwill Impairment (Details)	3 Months Ended			9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Mar. 31, 2014 USD ($) project MW	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Goodwill [Line Items]
Goodwill impairment expense	$ 0	$ 0		$ 0	$ 154,000,000
DPLER [Member]
Goodwill [Line Items]
Goodwill			$ 136,000,000
Goodwill implied fair value			0
Goodwill impairment expense			136,000,000
Buffalo Gap [Member]
Goodwill [Line Items]
Goodwill impairment expense			$ 18,000,000
Number of wind projects | project			3
Generation Capacity Mega Watts | MW			522