Commitments and Contingencies - Environmental Requirements - Water and Waste (Details) - Federal Coal Ash Regulation $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Site Contingency [Line Items]
Number of sites where regulated ash units will still be in operation at a specified date	3
Number of sites where statistically significant increases over established groundwater standards exist	0
Number of impoundments where closure plans will be expedited	1
Cost of construction of a new impoundment	$ 9
Estimated cost of closure of an impoundment	$ 4
Time period of existing ash pond closure completion in accordance with the CCR rule	5 years