Early Plant Retirements Early Plant Retirements - Implications of Potential Early Plant Retirement on Balance Sheet (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Implications of Potential Early Plant Retirement [Line Items]
Completed plant, net		$ 75,840	$ 74,202
Salem [Member] | Facility Closing [Member]
Implications of Potential Early Plant Retirement [Line Items]
Materials and Supplies Inventory		45
Nuclear Fuel Inventory, Net of Amortization		114
Completed plant, net		605
Construction Work in Progress		34
Asset Retirement Obligation		(455)
Mystic Generating Station [Member] | Facility Closing [Member]
Implications of Potential Early Plant Retirement [Line Items]
Materials and Supplies Inventory		21
Nuclear Fuel Inventory, Net of Amortization		18
Completed plant, net		877
Construction Work in Progress		5
Prepaid Expense	[1]	15
Asset Retirement Obligation		(5)
Accrued Expenses	[1]	$ (2)

[1] Reflects ending balances only as they relate to Mystic's Long-term Service Agreement.