Fair Value Measurements - Level 3 Quantitative Information (Details) - Level 3 - FTRs $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) $ / MWh	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	$ (1)	$ 3	$ 3
Model
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	$ (1)
Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH)	0.20
Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH)	1.90
Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH)	0.60