Retirement Benefits - Narrative (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 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		$ 5		$ 5	$ 16	$ 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		11	43	37
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]	67		54	200	193
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense		52		33	155	129
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		7	29	28
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		16		17	$ 49	50
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		2		2	$ 8	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	$ 8	$ 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 nine months ended September 30, 2015, the costs related to CENG were $5 million and $16 million, respectively. For the three months ended September 30, 2014, the costs related to CENG were $5 million. For the period of April 1, 2014 to September 30, 2014, the costs related to CENG were $11 million. CENG amounts are included in the table above.