Variable Interest Entities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) MW	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	$ 6,402,097	$ 6,082,456	$ 5,782,576
Limited Partnerships and Similar Entities [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	56,200
Variable Interest Entity Remaining Investment Commitment Amount	$ 30,700
Minimum [Member] | Limited Partnerships and Similar Entities [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Restrictions on Withdrawal of Member Capital Account	2021
Maximum [Member] | Limited Partnerships and Similar Entities [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Restrictions on Withdrawal of Member Capital Account	2040
Variable Interest Entity, Not Primary Beneficiary [Member] | Limited Partnerships and Similar Entities [Member] | VIE Commitments [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets	$ 51,200
Lancaster Power Purchase Agreement [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Evaluated Power Capacity | MW	270
Variable Interest Entity, Reporting Entity Involvement, Contractual Commitment, Amount	$ 175,100
Lancaster Power Purchase Agreement [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Minimum [Member]
Variable Interest Entity [Line Items]
Average service lives for the utility plan in service	15 years
Lancaster Power Purchase Agreement [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Average service lives for the utility plan in service	25 years