Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) (Exelon Generation Co L L C [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2014
Derivative [Line Items]
Investor-owned utilities, marketers and power producers	$ 264
Energy cooperative and municipalities	470
Financial institutions	749
Other	10
Net Exposure [Member]
Derivative [Line Items]
Investment grade	1,152
Non-investment grade	16
No external ratings - internally rated - investment grade	302
No external ratings - internally rated - non-investment grade	23
Total	1,493
Total Exposure Before Credit Collateral [Member]
Derivative [Line Items]
Investment grade	1,240
Non-investment grade	23
No external ratings - internally rated - investment grade	302
No external ratings - internally rated - non-investment grade	26
Total	1,591
Credit Collateral [Member]
Derivative [Line Items]
Investment grade	88
Non-investment grade	7
No external ratings - internally rated - investment grade	0
No external ratings - internally rated - non-investment grade	3
Total	98	[1]
Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Derivative [Line Items]
Investment grade	1
Non-investment grade	0
No external ratings - internally rated - investment grade	1
No external ratings - internally rated - non-investment grade	0
Total	2
Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Derivative [Line Items]
Investment grade	423
Non-investment grade	0
No external ratings - internally rated - investment grade	180
No external ratings - internally rated - non-investment grade	0
Total	$ 603

[1] As of September 30, 2014, credit collateral held from counterparties where Generation had credit exposure included $94 million of cash and $4 million of letters of credit.