Variable Interest Entities Variable Interest Entities (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) MW
Lancaster Power Purchase Agreement [Member]
Variable Interest Entity [Line Items]
Evaluated Power Capacity | MW	270
Maximum Loss Exposure, Amount	$ 289.9
Limited Partnerships and Similar Entities [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	$ 5.8
Minimum [Member] | Lancaster Power Purchase Agreement [Member]
Variable Interest Entity [Line Items]
Property, Plant and Equipment, Useful Life	15 years
Minimum [Member] | Limited Partnerships and Similar Entities [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Restrictions on Withdrawal of Member Capital Account	2016
Maximum [Member] | Lancaster Power Purchase Agreement [Member]
Variable Interest Entity [Line Items]
Property, Plant and Equipment, Useful Life	25 years
Maximum [Member] | Limited Partnerships and Similar Entities [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Restrictions on Withdrawal of Member Capital Account	2032