Fair Value Measurements (Details 1) - Level 3 - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
NUG contracts
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	$ 0	$ 1
Beginning Balance, Derivative Liabilities	(79)	(108)
Beginning Balance, Net	(79)	(107)
Unrealized gain (loss), Derivative Assets	0	0
Unrealized loss, Derivative Liabilities	(2)	(10)
Unrealized loss, Net	(2)	(10)
Purchases, Derivative Assets		0
Purchases, Derivative Liabilities		0
Purchases, Net		0
Settlements, Derivative Assets	0	(1)
Settlements, Derivative Liabilities	7	39
Settlements, Net	7	38
Ending Balance, Derivative Assets	0	0
Ending Balance, Derivative Liabilities	(74)	(79)
Ending Balance, Net	(74)	(79)
FTRs
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	3	3
Beginning Balance, Derivative Liabilities	0	(1)
Beginning Balance, Net	3	2
Unrealized gain (loss), Derivative Assets	1	1
Unrealized loss, Derivative Liabilities	0	(1)
Unrealized loss, Net	1	0
Purchases, Derivative Assets		3
Purchases, Derivative Liabilities		0
Purchases, Net		3
Settlements, Derivative Assets	(3)	(4)
Settlements, Derivative Liabilities	0	2
Settlements, Net	(3)	(2)
Ending Balance, Derivative Assets	1	3
Ending Balance, Derivative Liabilities	0	0
Ending Balance, Net	$ 1	$ 3