Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) (Exelon Generation Co L L C [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Total Exposure Before Credit Collateral [Member]
Derivative [Line Items]
Investment grade	$ 1,629
Non-investment grade	49
Internally rated—investment grade	479
Internally rated—non-investment grade	60
Total	2,217
Credit Collateral [Member]
Derivative [Line Items]
Investment grade	62
Non-investment grade	19
Internally rated—investment grade	0
Internally rated—non-investment grade	4
Total	85
Net Exposure [Member]
Derivative [Line Items]
Investment grade	1,567
Non-investment grade	30
Internally rated—investment grade	479
Internally rated—non-investment grade	56
Total	2,132
Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Derivative [Line Items]
Investment grade	1
Non-investment grade	0
Internally rated—investment grade	0
Internally rated—non-investment grade	0
Total	1
Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Derivative [Line Items]
Investment grade	452
Non-investment grade	0
Internally rated—investment grade	0
Internally rated—non-investment grade	0
Total	$ 452