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, 2018 USD ($)
Derivative [Line Items]
Financial institutions	$ 12
Investor-owned utilities, marketers, power producers	737
Energy cooperatives and municipalities	324
Other	76
Total	$ 1,149