Retirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Jun. 30, 2012	Mar. 31, 2012
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	$ 267			$ 55
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	22			16
Exelon Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Other comprehensive income (loss) due to curtailment	(75)
Regulatory asset increase (decrease) due to updated valuation adjustment	93
Business Services Company [Member]
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	17	[1]		14	[1]
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	2			1
Exelon Generation Co L L C [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	115			20
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	87			81
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	11			8
Commonwealth Edison Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	118			9
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	77			69
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	5			4
PECO Energy Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	11			5
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	11			13
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	2			2
Baltimore Gas and Electric Company [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	5			7
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	13	[2]		16	[2]
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	2	[3]		2	[3]
Pension Plans Defined Benefit [Member]
Change in benefit obligation:
Service cost	80			61
Interest cost	163			164
Components of net periodic benefit cost:
Service cost	80			61
Interest cost	163			164
Expected return on assets	(253)			(232)
Amortization of:
Transition obligation	0			0
Prior service cost (credit)	3			4
Actuarial (loss) gain	140		106
Contractual termination benefit cost	0
Net periodic benefit cost	133			103
Pension Plans Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Benefit obligation increase (decrease) reflecting actual census data	8
Other Postretirement Benefit Plans Defined Benefit [Member]
Change in benefit obligation:
Service cost	41			37
Interest cost	48			51
Components of net periodic benefit cost:
Service cost	41			37
Interest cost	48			51
Expected return on assets	(33)			(29)
Amortization of:
Transition obligation	0			3
Prior service cost (credit)	(4)			(3)
Actuarial (loss) gain	20			19
Curtailments charges	0
Contractual termination benefit cost	0
Net periodic benefit cost	72			78
Other Postretirement Benefit Plans Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Benefit obligation increase (decrease) reflecting actual census data	$ (39)

[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] BGE's pension and postretirement benefit costs for the three months ended March 31, 2012 include $12 million of costs incurred prior to the closing of Exelon's merger with Constellation on March 12, 2012. These amounts are not included in Exelon's net periodic benefit costs for the three months ended March 31, 2012 shown in the first table of the Defined Benefit Pension and Other Postretirement Benefits section above.

[3] BGE's matching contributions for the three months ended March 31, 2012 include $1 million of costs incurred prior to the closing of Exelon's merger with Constellation on March 12, 2012, which is not included in Exelon's matching contributions for the three months ended March 31, 2012.