Retirement Benefits - Allocated Portion of Pension and Postretirement Benefit Plan Costs (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense	[1]	$ 68	$ 63	$ 133	$ 139
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense		51	40	103	96
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense		10	9	19	21
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense		17	17	33	33
Business Services Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense	[2]	14	13	29	$ 26
CENG [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense		6	5	11
CENG [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense		$ 3	$ 2	$ 5

[1] For the three and six months ended June 30, 2015, the costs related to CENG were $6 million and $11 million, respectively. For the period of April 1, 2014 to June 30, 2014, amounts include $5 million related to CENG.

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