Derivative Financial Instruments Schedule of Notional Amounts Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 636	$ 636
Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	800	800
Exelon Generation Co L L C [Member] | Foreign Exchange Contract [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 87	$ 94