Revenue from Contracts with Customers Revenue from Contracts with Customers - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Capitalized Contract Cost [Line Items]
Capitalized Contract Cost, Gross	$ 30		$ 30		$ 26
Capitalized Contract Cost, Amortization	6	$ 7	16	$ 24
Exelon Generation Co L L C [Member]
Capitalized Contract Cost [Line Items]
Capitalized Contract Cost, Gross	30		30		$ 26
Capitalized Contract Cost, Amortization	$ 6	$ 7	$ 16	$ 24