Asset Sale and Exit Activities - Narrative (Details) - Retention Benefits - D.E. Karn Generating Complex $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Expected cost	$ 35	$ 35	$ 35
Retention costs	$ 4	11	14
Costs incurred and capitalized		$ 1
Property, Plant and Equipment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Costs incurred and capitalized			$ 2