Fair Value Measurements - Level 3 Rollforward (Details) - Level 3 - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
NUG contracts
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	$ 0	$ 0
Beginning Balance, Derivative Liabilities	(16)	(44)
Beginning Balance, Net	(16)	(44)
Unrealized gain (loss), Derivative Assets	0	0
Unrealized loss, Derivative Liabilities	(3)	(11)
Unrealized loss, Net	(3)	(11)
Purchases, Derivative Assets	0	0
Purchases, Derivative Liabilities	0	0
Purchases, Net	0	0
Settlements, Derivative Assets	0	0
Settlements, Derivative Liabilities	13	39
Settlements, Net	13	39
Ending Balance, Derivative Assets	0	0
Ending Balance, Derivative Liabilities	(6)	(16)
Ending Balance, Net	(6)	(16)
FTRs
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	4	10
Beginning Balance, Derivative Liabilities	(1)	(1)
Beginning Balance, Net	3	9
Unrealized gain (loss), Derivative Assets	0	(1)
Unrealized loss, Derivative Liabilities	0	0
Unrealized loss, Net	0	(1)
Purchases, Derivative Assets	0	6
Purchases, Derivative Liabilities	0	(4)
Purchases, Net	0	2
Settlements, Derivative Assets	(4)	(11)
Settlements, Derivative Liabilities	0	4
Settlements, Net	(4)	(7)
Ending Balance, Derivative Assets	0	4
Ending Balance, Derivative Liabilities	(1)	(1)
Ending Balance, Net	$ (1)	$ 3