Derivative Financial Instruments - Net Credit Exposure by Type of Counterparty (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Cash collateral held	$ 13
Letters of credit held	69
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Cash collateral held	21	$ 77
Letters of credit held	78	$ 24
Exelon Generation Co L L C [Member] | Net Exposure [Member]
Derivative [Line Items]
Financial institutions	187
Investor-owned utilities, marketers, power producers	886
Energy cooperatives and municipalities	872
Other	35
Total	$ 1,980