Variable Interest Entities (Narrative) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($) MW	Dec. 31, 2020 USD ($)
Variable Interest Entity [Line Items]
Investments	$ 73	$ 70
Variable Interest Entity, Primary Beneficiary | Aviator Wind
Variable Interest Entity [Line Items]
Ownership interest	51.00%
Nameplate capacity (in MW) | MW	525
Variable Interest Entity, Primary Beneficiary | Aviator Wind Class B Membership
Variable Interest Entity [Line Items]
Noncontrolling ownership interest	49.00%
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Investments	$ 73	$ 70
Variable Interest Entity, Not Primary Beneficiary | T.E.S. Filer City
Variable Interest Entity [Line Items]
Ownership interest	50.00%
Variable Interest Entity, Not Primary Beneficiary | Grayling
Variable Interest Entity [Line Items]
Ownership interest	50.00%
Variable Interest Entity, Not Primary Beneficiary | Genesee
Variable Interest Entity [Line Items]
Ownership interest	50.00%
Variable Interest Entity, Not Primary Beneficiary | Craven
Variable Interest Entity [Line Items]
Ownership interest	50.00%