Derivative Financial Instruments - Net Credit Exposure by Type of Counterparty (Details) - Exelon Generation Co L L C [Member] - Net Exposure [Member] $ in Millions	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Financial institutions	$ 41
Investor-owned utilities, marketers, power producers	558
Energy cooperatives and municipalities	452
Other	86
Total	$ 1,137