Significant Accounting Policies - Summary of Capitalized Software Costs (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Capitalized Software [Line Items]
Net unamortized software costs	$ 596	[1]	$ 479	[1]
Amortization of capitalized software costs	186	[1],[2]	198	[1],[2]	208	[1],[2]
Exelon Generation Co L L C [Member]
Capitalized Software [Line Items]
Net unamortized software costs	193	[1]	129	[1]
Amortization of capitalized software costs	59	[1],[2]	67	[1],[2]	81	[1],[2]
Commonwealth Edison Co [Member]
Capitalized Software [Line Items]
Net unamortized software costs	133		101
Amortization of capitalized software costs	45		52		56
PECO Energy Co [Member]
Capitalized Software [Line Items]
Net unamortized software costs	84		71
Amortization of capitalized software costs	28		33		30
Baltimore Gas and Electric Company [Member]
Capitalized Software [Line Items]
Net unamortized software costs	163		155
Amortization of capitalized software costs	$ 43	[2]	$ 36	[2]	$ 32	[2]

[1] 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.

[2] 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.