Fair Value of Financial Assets and Liabilities - Commodity Derivatives (Details) MWh in Millions, MMBTU in Millions, $ in Millions		Mar. 31, 2021 USD ($) MWh MMBTU	Dec. 31, 2020 MWh MMBTU
cash flow hedge commodity [Member]
Derivative [Line Items]
Commodity contracts designated as cash flow hedges | $		$ 0
Electric Commodity (in megawatt hours)
Derivative [Line Items]
Derivative, Nonmonetary Notional amount | MWh	[1],[2]	22	17
Natural Gas Commodity (in million British thermal units)
Derivative [Line Items]
Derivative, Nonmonetary Notional amount | MMBTU	[1],[2]	86	93

[1] Amounts are not reflective of net positions in the underlying commodities.

[2] Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.