Variable Interest Entities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) MW
Lancaster Power Purchase Agreement [Member]
Variable Interest Entity [Line Items]
Evaluated Power Capacity | MW	270
Variable Interest Entity, Reporting Entity Involvement, Contractual Commitment, Amount	$ 260.2
Limited Partnerships and Similar Entities [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	$ 12.2
Minimum [Member] | Lancaster Power Purchase Agreement [Member]
Variable Interest Entity [Line Items]
Average service lives for the utility plan in service	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	2019
Maximum [Member] | Lancaster Power Purchase Agreement [Member]
Variable Interest Entity [Line Items]
Average service lives for the utility plan in service	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	2037
Sixth VIE [Member] | Limited Partnerships and Similar Entities [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Reporting Entity Involvement, Contractual Commitment, Amount	$ 25.0
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	9.7
Variable Interest Entity Remaining Investment Commitment Amount	$ 15.3