Retirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions					$ 360		$ 131
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	18		19		61		55
Exelon Legacy Benefit Plans [Member]
Changes in plan assets and benefit obligations recognized in OCI and regulatory assets:
Changes in plan assets and benefit obligations recognized in OCI					(75)
Changes in plan assets and benefit obligations recognized in regulatory assets					93
Constellation Legacy Benefit Plans [Member]
Changes in plan assets and benefit obligations recognized in OCI and regulatory assets:
Changes in plan assets and benefit obligations recognized in OCI					2
Changes in plan assets and benefit obligations recognized in regulatory assets					14
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	24	[1]	20	[1]	58	[1]	63	[1]
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	1		2		5		6
Exelon Generation Co L L C [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions					123		48
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	87		85		259		259
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	8		9		29		25
Commonwealth Edison Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions					120		19
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	77		75		231		212
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	6		5		16		14
PECO Energy Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions					10		12
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	11		12		32		38
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	2		2		6		5
Baltimore Gas and Electric Company [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions					16		13
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	14		14		41	[2]	46	[2]
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	1	[3]	1	[3]	5	[3]	5	[3]
Pension Plans Defined Benefit [Member]
Change in benefit obligation:
Service cost	79		76		238		211
Interest cost	163		181		488		524
Settlements			(9)				(9)
Change in plan assets:
Settlements			(9)				(9)
Components of net periodic benefit cost:
Service cost	79		76		238		211
Interest cost	163		181		488		524
Expected return on assets	(253)		(258)		(761)		(742)
Amortization of:
Prior service cost (credit)	3		5		10		12
Actuarial (loss) gain	140		117		421		338
Settlement charges	9				9
Contractual termination benefit cost							14	[4]
Net periodic benefit cost	141		130		405		366
Pension Plans Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Change in plan assets:
Fair value of net plan assets at end of year	8				8
Pension Plans Defined Benefit [Member] | Constellation Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Benefit obligation increase (decrease) reflecting actual census data					23
Other Postretirement Benefit Plans Defined Benefit [Member]
Change in benefit obligation:
Service cost	41		38		122		114
Interest cost	48		53		145		157
Components of net periodic benefit cost:
Service cost	41		38		122		114
Interest cost	48		53		145		157
Expected return on assets	(33)		(28)		(99)		(86)
Amortization of:
Transition obligation			2				8
Prior service cost (credit)	(4)		(3)		(14)		(10)
Actuarial (loss) gain	20		19		62		58
Curtailments charges			(5)				(7)
Contractual termination benefit cost							6	[4]
Net periodic benefit cost	72		76		216		240
Other Postretirement Benefit Plans Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Change in plan assets:
Fair value of net plan assets at end of year	(39)				(39)
Other Postretirement Benefit Plans Defined Benefit [Member] | Constellation Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Benefit obligation increase (decrease) reflecting actual census data					$ (12)

[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. As of September 30, 2012, ComEd and BGE each recorded a regulatory asset of $1 million related to their BSC-billed portion of the second quarter 2012 contractual termination benefit charge.

[2] BGE's pension and postretirement benefit costs for the nine months ended September 30, 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 nine months ended September 30, 2012 shown in the first table of the Defined Benefit Pension and Other Postretirement Benefits section above.

[3] BGE's matching contributions for the nine months ended September 30, 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 nine months ended September 30, 2012.

[4] ComEd and BGE established regulatory assets of $1 million and $4 million, respectively, for their portion of the second quarter 2012 contractual termination benefit charge.