Exit Activities - Narrative (Details) - Retention Benefits - D.E. Karn Generating Complex - USD ($) $ in Millions	3 Months Ended		18 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Expected cost	$ 35		$ 35
Retention and severance costs	0	$ 4	16
Retention Incentive Program
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Regulatory asset	$ 2		2
Property, Plant and Equipment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Costs incurred and capitalized			$ 3