Significant Accounting Policies - Summary of Total Interest Incurred, Capitalized and Credits to AFUDC (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Capitalized Interest And AFUDC [Line Items]
Total interest incurred	$ 1,144	[1],[2],[3]	$ 1,423	[1],[2],[3]	$ 1,003	[1],[2],[3]
Capitalized interest	63	[1],[2]	54	[1],[2]	67	[1],[2]
Credits to AFUDC debt and equity	37	[1],[2]	35	[1],[2]	25	[1],[2]
Exelon Generation Co L L C [Member]
Capitalized Interest And AFUDC [Line Items]
Total interest incurred	419	[1],[2],[3]	411	[1],[2],[3]	368	[1],[2],[3]
Capitalized interest	63	[1],[2]	54	[1],[2]	67	[1],[2]
Credits to AFUDC debt and equity	0	[1],[2]	0	[1],[2]	0	[1],[2]
Commonwealth Edison Co [Member]
Capitalized Interest And AFUDC [Line Items]
Total interest incurred	323	[3]	584	[3]	310	[3]
Capitalized interest	0		0		0
Credits to AFUDC debt and equity	5		16		9
PECO Energy Co [Member]
Capitalized Interest And AFUDC [Line Items]
Total interest incurred	115	[3]	117	[3]	125	[3]
Capitalized interest	0		0		0
Credits to AFUDC debt and equity	8		6		6
Baltimore Gas and Electric Company [Member]
Capitalized Interest And AFUDC [Line Items]
Total interest incurred	118	[1],[3]	129	[1],[3]	149	[1],[3]
Capitalized interest	0	[1]	0	[1]	0	[1]
Credits to AFUDC debt and equity	$ 24	[1]	$ 13	[1]	$ 15	[1]

[1] Exelon activity for the year ended December 31, 2012 includes the results of Constellation and BGE for March 12, 2012—December 31, 2012. Generation activity for the year ended December 31, 2012 includes the results of Constellation for March 12, 2012—December 31, 2012. BGE activity represents the activity for the year ended December 31, 2012.

[2] On April 1, 2014, Generation assumed operational control of CENG’s nuclear fleet. As a result, the 2014 financial results include CENG’s financial position and results of operations beginning April 1, 2014.

[3] Includes interest expense to affiliates.