Fair Value Measurements (Details 1) - Level 3 - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017
NUG contracts
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	$ 0
Beginning Balance, Derivative Liabilities	(44)
Beginning Balance, Net	(44)
Unrealized gain (loss), Derivative Assets	0	$ 0
Unrealized loss, Derivative Liabilities	(7)	2
Unrealized loss, Net	(7)	2
Purchases, Derivative Assets	0	0
Purchases, Derivative Liabilities	0	0
Purchases, Net	0	0
Settlements, Derivative Assets	0	0
Settlements, Derivative Liabilities	10	33
Settlements, Net	10	33
Ending Balance, Derivative Assets	0	0
Ending Balance, Derivative Liabilities	(41)	(79)
Ending Balance, Net	(41)	(79)
FTRs
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	10
Beginning Balance, Derivative Liabilities	(1)
Beginning Balance, Net	9
Unrealized gain (loss), Derivative Assets	0	8
Unrealized loss, Derivative Liabilities	0	1
Unrealized loss, Net	0	9
Purchases, Derivative Assets	0	5
Purchases, Derivative Liabilities	0	(5)
Purchases, Net	0	0
Settlements, Derivative Assets	(10)	(6)
Settlements, Derivative Liabilities	0	3
Settlements, Net	(10)	(3)
Ending Balance, Derivative Assets	0	3
Ending Balance, Derivative Liabilities	(1)	0
Ending Balance, Net	$ (1)	$ 3