Fair Value Measurements - Level 3 Rollforward (Details) - Level 3 - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
NUG contracts
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Liabilities	$ 0	$ (16)
Unrealized loss, Derivative Liabilities	0	(3)
Purchases, Derivative Liabilities		0
Settlements, Derivative Liabilities	0	19
Ending Balance, Derivative Liabilities	0	0
FTRs
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	3	4
Beginning Balance, Derivative Liabilities	0	(1)
Beginning Balance, Net	3	3
Unrealized loss, Derivative Assets	0	(3)
Unrealized loss, Derivative Liabilities	(1)	0
Unrealized loss, Net	(1)	(3)
Purchases, Derivative Assets		7
Purchases, Derivative Liabilities		(2)
Purchases, Net		5
Settlements, Derivative Assets	(3)	(5)
Settlements, Derivative Liabilities	0	3
Settlements, Net	(3)	(2)
Ending Balance, Derivative Assets	0	3
Ending Balance, Derivative Liabilities	(1)	0
Ending Balance, Net	$ (1)	$ 3