Fair Value Measurements - Level 3 Rollforward (Details) - Level 3 - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
NUG contracts
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	$ 0	$ 0
Beginning Balance, Derivative Liabilities	(44)	(79)
Beginning Balance, Net	(44)	(79)
Unrealized gain (loss), Derivative Assets	0	0
Unrealized loss, Derivative Liabilities	(11)	2
Unrealized loss, Net	(11)	2
Purchases, Derivative Assets	0	0
Purchases, Derivative Liabilities	0	0
Purchases, Net	0	0
Settlements, Derivative Assets	0	0
Settlements, Derivative Liabilities	20	33
Settlements, Net	20	33
Ending Balance, Derivative Assets	0	0
Ending Balance, Derivative Liabilities	(35)	(44)
Ending Balance, Net	(35)	(44)
FTRs
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	10	3
Beginning Balance, Derivative Liabilities	(1)	0
Beginning Balance, Net	9	3
Unrealized gain (loss), Derivative Assets	0	8
Unrealized loss, Derivative Liabilities	0	1
Unrealized loss, Net	0	9
Purchases, Derivative Assets	6	5
Purchases, Derivative Liabilities	(4)	(5)
Purchases, Net	2	0
Settlements, Derivative Assets	(10)	(6)
Settlements, Derivative Liabilities	1	3
Settlements, Net	(9)	(3)
Ending Balance, Derivative Assets	6	10
Ending Balance, Derivative Liabilities	(4)	(1)
Ending Balance, Net	$ 2	$ 9