Retirement Benefits - Narrative (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Changes in plan assets and benefit obligations recognized in OCI			$ 27
Increase in regulatory assets due valuation received by Exelon for its legacy pension and other postretirement benefit obligations.			48
Increase in regulatory liabilities due to updated valuation of Exelon's legacy pension and postretirement benefit obligations			11
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
Business Services Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense	[1]	14		13	29	$ 26
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	[2]	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
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation increase (decrease) reflecting actual census data			45
Expected return on assets					7.00%
Discount rate					3.94%
Pension Plan, Defined Benefit [Member] | CENG [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense		3		2	$ 5
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation increase (decrease) reflecting actual census data			$ 57
Expected return on assets					6.46%
Discount rate					3.92%
Other Postretirement Benefit Plan, Defined Benefit [Member] | CENG [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense		$ 3		$ 3	$ 6

[1] 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.

[2] 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.