Retirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions					$ 284		$ 90
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	21		19		43		35
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	16	[1]	25	[1]	34	[1]	42	[1]
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	2		3		4		4
Exelon Generation Co L L C [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions					120		35
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	87		94		173		175
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	10		8		21		16
Commonwealth Edison Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions					119		12
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	77		68		154		137
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	5		5		10		9
PECO Energy Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions					10		8
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	10		13		21		26
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	2		2		4		4
Baltimore Gas and Electric Company [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions					11		8
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	14		18		27	[2]	32	[2]
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	2	[3]	1	[3]	4	[3]	3	[3]
Pension Plans Defined Benefit [Member]
Change in benefit obligation:
Service cost	79		74		159		135
Interest cost	163		179		326		343
Components of net periodic benefit cost:
Service cost	79		74		159		135
Interest cost	163		179		326		343
Expected return on assets	(254)		(253)		(508)		(484)
Amortization of:
Transition obligation					0		0
Prior service cost (credit)	4		3		7		7
Actuarial (loss) gain	141		115		281		221
Curtailments charges			0		0		0
Settlement charges	0				0
Contractual termination benefit cost			14	[4]	0	[4]	14	[4]
Net periodic benefit cost	133		132		265		236
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	40		39		81		76
Interest cost	49		54		97		104
Components of net periodic benefit cost:
Service cost	40		39		81		76
Interest cost	49		54		97		104
Expected return on assets	(33)		(29)		(66)		(58)
Amortization of:
Transition obligation	0		2		0		6
Prior service cost (credit)	(6)		(3)		(10)		(6)
Actuarial (loss) gain	21		19		42		38
Curtailments charges	0		(2)		0		(2)
Contractual termination benefit cost			6	[4]	0	[4]	6	[4]
Net periodic benefit cost	71		86		144		164
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 June 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 six months ended June 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 six months ended June 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 six months ended June 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 six months ended June 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.