Common Stock - Schedule of Pre-Tax Stock-based Compensation Expense (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax stock-based compensation expense	$ 127		$ 118		$ 115	[1]
Exelon Generation Co L L C [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax stock-based compensation expense	52		48		42	[1]
Commonwealth Edison Co [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax stock-based compensation expense	7		9		11	[1]
PECO Energy Co [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax stock-based compensation expense	3		5		5	[1]
Baltimore Gas and Electric Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax stock-based compensation expense	5		6		5	[1]
Business Services Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax stock-based compensation expense	60	[2]	50	[2]	52	[1],[2]
CENG [Member] | Baltimore Gas and Electric Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax stock-based compensation expense					$ 2

[1] BGE’s stock-based compensation expense (pre-tax) for December 31, 2012 excludes $2 million of cost incurred in 2012 prior to the closing of Exelon’s merger with Constellation on March 12, 2012. This amount is not included in Exelon’s stock-based compensation expense for the year ended December 31, 2012 shown in the table titled Components of Stock-Based Compensation Expense and the breakout by subsidiary above.

[2] These amounts primarily represent amounts billed to Exelon’s subsidiaries through intercompany allocations. These amounts are not included in the Generation, ComEd, PECO and BGE amounts above.