Retirement Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Defined Benefit Plan Effect Of One Percentage Point Change In Assumed Health Care Cost Trend Rates [Abstract]
Effect of a one percentage point increase in assumed healthcare cost trend on 2010 total service and interest cost components	$ 81.00
Effect of a one percentage point increase in assumed healthcare cost trend on postretirement benefit obligation at December 31, 2010	845.00
Effect of a one percentage point decrease in assumed healthcare cost trend on 2010 total service and interest cost components	56.00
Effect of a one percentage point decrease in assumed healthcare cost trend on postretirement benefit obligation at December 31, 2010	569.00
Change in benefit obligation:
Acquisitions/divestitures	362.00
Settlements	(169.00)
Reinsurance proceeds received	1.30
Change in plan assets:
Benefits paid net			(189.00)
Settlements	(169.00)
Prescription Drug Benefit Effect Of Subsidy On Net Periodic Postretirement Benefit Cost [Abstract]
Amortization of the actuarial experience loss	(17.00)		3.00		9.00
Reduction in current period service cost	0		9.00		10.00
Reduction in interest cost on the APBO	0		16.00		19.00
Effect of federal subsidy on net periodic postretirement benefit costs under the Prescription Drug Act	(17.00)		28.00		38.00
Changes in plan assets and benefit obligations recognized in OCI and regulatory assets:
Settlements	(31.00)
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	462.00		2,360.00		959.00
Federal subsidy payments received not included in contributions	10.00		11.00		10.00
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	820.00		542.00		581.00
Defined Benefit Plan Weighted Average Asset Allocations [Abstract]
Target asset allocation percentage			32.00%
Total weighted average asset allocation	40.00%
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	67.00		78.00		81.00
Projected Benefit Obligation In Excess Of Plan Assets [Member]
Defined Benefit Plan Pension Plans With Accumulated Benefit Obligations In Excess Of Plan Assets [Abstract]
Projected benefit obligation	16,800.00		13,538.00
Fair value of net plan assets	13,357.00		11,302.00
Accumulated Benefit Obligation In Excess Of Plan Assets [Member]
Defined Benefit Plan Pension Plans With Accumulated Benefit Obligations In Excess Of Plan Assets [Abstract]
Projected benefit obligation	16,796.00		13,538.00
Accumulated benefit obligation	15,657.00		12,616.00
Fair value of net plan assets	13,353.00		11,302.00
Equity Securities [Member]
Defined Benefit Plan Weighted Average Asset Allocations [Abstract]
Total weighted average asset allocation			37.00%
Fixed Income Securities [Member]
Defined Benefit Plan Weighted Average Asset Allocations [Abstract]
Total weighted average asset allocation	40.00%		53.00%
Alternative Investments [Member]
Defined Benefit Plan Weighted Average Asset Allocations [Abstract]
Total weighted average asset allocation	14.00%		10.00%
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	99.00	[1]	48.00	[1]	52.00	[1]
Exelon Generation Co L L C [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	178.00		1,070.00		445.00
Federal subsidy payments received not included in contributions	5.00		5.00		5.00
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	341.00		249.00		268.00
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	30.00		40.00		42.00
Commonwealth Edison Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	138.00		977.00		317.00
Federal subsidy payments received not included in contributions	4.00		4.00		3.00
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	282.00		213.00		215.00
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	19.00		22.00		22.00
PECO Energy Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	45.00		137.00		106.00
Federal subsidy payments received not included in contributions	1.00		1.00		2.00
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	50.00		32.00		46.00
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	7.00		9.00		9.00
Baltimore Gas and Electric Company [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	16.00		67.00		214.00
Federal subsidy payments received not included in contributions	2.00		3.00		2.00
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	60.00		51.00		48.00
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	7.00		7.00		6.00
Pension Plans Defined Benefit [Member]
Change in benefit obligation:
Net benefit obligation at beginning of year	13,538.00		12,524.00
Service cost	280.00		212.00		190.00
Interest cost	698.00		649.00		660.00
Actuarial gain (loss)	1,520.00		807.00
Acquisitions/divestitures	1,880.00
Curtailments	(10.00)
Settlements	(169.00)
Special termination benefits	15.00
Gross benefits paid	(952.00)		(654.00)
Net benefit obligation at end of year	16,800.00		13,538.00		12,524.00
Change in plan assets:
Fair value of net plan assets at beginning of year	11,302.00		8,859.00
Actual return on plan assets	1,484.00		1,003.00
Employer contributions	149.00		2,094.00
Gross benefits paid	(952.00)		(654.00)
Settlements	(169.00)
Fair value of net plan assets at end of year	13,357.00		11,302.00		8,859.00
Components of net periodic benefit cost:
Service cost	280.00		212.00		190.00
Interest cost	698.00		649.00		660.00
Expected return on assets	(988.00)		(939.00)		(799.00)
Amortization of:
Prior service cost (credit)	15.00		14.00		14.00
Actuarial (loss) gain	450.00		331.00		254.00
Curtailments charges	0		0		0
Settlement charges	(31.00)				(5.00)
Contractual termination benefit cost	(14.00)
Net periodic benefit cost	500.00		267.00		324.00
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Other current liabilities	15.00		42.00
Pension obligations	3,428.00		2,194.00
Unfunded status (net benefit obligation less net plan assets)	3,443.00		2,236.00
Changes in plan assets and benefit obligations recognized in OCI and regulatory assets:
Current year actuarial (gain) loss	1,693.00		744.00		737.00
Amortization of actuarial gain (loss)	(450.00)		(331.00)		(254.00)
Current year prior service cost	1.00		0		0
Amortization of prior service cost	(15.00)		(14.00)		(14.00)
Curtailments	(10.00)
Settlements					(5.00)
Total recognized in OCI and regulatory assets	1,188.00	[2]	399.00	[2]	464.00	[2]
Changes in plan assets and benefit obligations recognized in OCI	283.00		181.00		310.00
Changes in plan assets and benefit obligations recognized in regulatory assets	904.00		218.00		154.00
Defined Benefit Plan Accumulated Other Comprehensive Income And Regulatory Assets Before Tax [Abstract]
Prior service cost (credit)	76.00		90.00
Actuarial loss	7,931.00		6,729.00
Total components of periodic benefit cost not yet recognized	8,007.00	[3]	6,819.00	[3]
Benefits included in accumulated other comprehensive income	4,594.00		4.00
Benefits included in regulatory assets	3,413.00		2.00
Defined Benefit Plan Accumulated Other Comprehensive Income And Regulatory Assets To Be Amortized Before Tax [Abstract]
Prior service cost (credit)	14.00
Actuarial loss	568.00
Total components of periodic benefit cost to be amortized	582.00	[4]
Benefits included in accumulated other comprehensive income	315.00
Benefits included in regulatory assets	267.00
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	149.00		2,094.00		766.00
Defined Benefit Plan Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan Expected Future Benefit Payments In Year One	943.00
Defined Benefit Plan Expected Future Benefit Payments In Year Two	807.00
Defined Benefit Plan Expected Future Benefit Payments In Year Three	891.00
Defined Benefit Plan Expected Future Benefit Payments In Year Four	868.00
Defined Benefit Plan Expected Future Benefit Payments In Year Five	902.00
Defined Benefit Plan Expected Future Benefit Payments In Five Fiscal Years Thereafter	5,161.00
Total estimated future benefits payments through 2020	9,572.00
Pension Plans Defined Benefit [Member] | Equity Securities [Member]
Defined Benefit Plan Weighted Average Asset Allocations [Abstract]
Target asset allocation percentage	34.00%
Total weighted average asset allocation	35.00%
Pension Plans Defined Benefit [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Weighted Average Asset Allocations [Abstract]
Target asset allocation percentage	40.00%		47.00%
Pension Plans Defined Benefit [Member] | Alternative Investments [Member]
Defined Benefit Plan Weighted Average Asset Allocations [Abstract]
Target asset allocation percentage	26.00%		21.00%
Total weighted average asset allocation	25.00%
Pension Plans Defined Benefit [Member] | Business Services Company [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	63.00		157.00		77.00
Pension Plans Defined Benefit [Member] | Exelon Generation Co L L C [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	48.00		954.00		356.00
Pension Plans Defined Benefit [Member] | Commonwealth Edison Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	25.00		873.00		260.00
Pension Plans Defined Benefit [Member] | PECO Energy Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	13.00		110.00		73.00
Pension Plans Defined Benefit [Member] | Baltimore Gas and Electric Company [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions			54.00		197.00
Other Postretirement Benefit Plans Defined Benefit [Member]
Change in benefit obligation:
Net benefit obligation at beginning of year	4,062.00		3,874.00
Service cost	156.00		142.00		124.00
Interest cost	205.00		207.00		214.00
Plan participants' contributions	34.00		25.00
Actuarial gain (loss)	313.00		4.00
Plan amendments	(103.00)
Curtailments	(8.00)
Special termination benefits	(6.00)		0		(1.00)
Gross benefits paid	(219.00)		(201.00)
Federal subsidy on benefits paid	12.00		11.00
Net benefit obligation at end of year	4,820.00		4,062.00		3,874.00
Reinsurance proceeds received			12.00
Change in plan assets:
Fair value of net plan assets at beginning of year	1,797.00		1,655.00
Actual return on plan assets	197.00		29.00
Employer contributions	325.00		277.00
Plan participants' contributions	34.00		25.00
Gross benefits paid	(219.00)		(201.00)
Benefits paid net	(218.00)
Fair value of net plan assets at end of year	2,135.00		1,797.00		1,655.00
Components of net periodic benefit cost:
Service cost	156.00		142.00		124.00
Interest cost	205.00		207.00		214.00
Expected return on assets	(115.00)		(111.00)		(109.00)
Amortization of:
Transition obligation	11.00		9.00		9.00
Prior service cost (credit)	(17.00)		(38.00)		(56.00)
Actuarial (loss) gain	81.00		66.00		74.00
Curtailments charges	7.00
Net periodic benefit cost	320.00		275.00		257.00
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Other current liabilities	23.00		2.00
Non-pension postretirement benefit obligations	2,662.00		2,263.00
Unfunded status (net benefit obligation less net plan assets)	2,685.00		2,265.00
Changes in plan assets and benefit obligations recognized in OCI and regulatory assets:
Current year actuarial (gain) loss	304.00		74.00		0
Amortization of actuarial gain (loss)	(81.00)		(66.00)		(74.00)
Current year prior service cost	(109.00)
Amortization of prior service cost	17.00		38.00		56.00
Current year transition (asset) obligation	(1.00)
Amortization of transition obligation	(11.00)		(9.00)		(9.00)
Curtailments	(1.00)
Total recognized in OCI and regulatory assets	120.00	[2]	37.00	[2]	(27.00)	[2]
Changes in plan assets and benefit obligations recognized in OCI	39.00		13.00		(9.00)
Changes in plan assets and benefit obligations recognized in regulatory assets	81.00		24.00		(18.00)
Defined Benefit Plan Accumulated Other Comprehensive Income And Regulatory Assets Before Tax [Abstract]
Transition obligation	0		11.00
Prior service cost (credit)	(107.00)		(16.00)
Actuarial loss	1,185.00		963.00
Total components of periodic benefit cost not yet recognized	1,078.00	[3]	958.00	[3]
Benefits included in accumulated other comprehensive income	514.00		4.00
Benefits included in regulatory assets	564.00		4.00
Defined Benefit Plan Accumulated Other Comprehensive Income And Regulatory Assets To Be Amortized Before Tax [Abstract]
Transition obligation	0
Prior service cost (credit)	(19.00)
Actuarial loss	84.00
Total components of periodic benefit cost to be amortized	65.00	[4]
Benefits included in accumulated other comprehensive income	29.00
Benefits included in regulatory assets	36.00
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	323.00	[5]	277.00	[5]	203.00	[5]
Defined Benefit Plan Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan Expected Future Benefit Payments In Year One	197.00
Defined Benefit Plan Expected Future Benefit Payments In Year Two	204.00
Defined Benefit Plan Expected Future Benefit Payments In Year Three	212.00
Defined Benefit Plan Expected Future Benefit Payments In Year Four	220.00
Defined Benefit Plan Expected Future Benefit Payments In Year Five	231.00
Defined Benefit Plan Expected Future Benefit Payments In Five Fiscal Years Thereafter	1,330.00
Total estimated future benefits payments through 2020	2,394.00
Disclosure Of Expected Gross Prescription Drug Subsidy Receipts [Abstract]
Prescription Drug Subsidy Receipts Year One	0
Prescription Drug Subsidy Receipts Year Two	0
Prescription Drug Subsidy Receipts Year Three	0
Prescription Drug Subsidy Receipts Year Four	0
Prescription Drug Subsidy Receipts Year Five	0
Prescription Drug Subsidy Receipts Five Fiscal Years Thereafter	0
Other Postretirement Benefit Plans Defined Benefit [Member] | Equity Securities [Member]
Defined Benefit Plan Weighted Average Asset Allocations [Abstract]
Target asset allocation percentage	45.00%
Total weighted average asset allocation	46.00%
Other Postretirement Benefit Plans Defined Benefit [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Weighted Average Asset Allocations [Abstract]
Target asset allocation percentage	40.00%
Other Postretirement Benefit Plans Defined Benefit [Member] | Alternative Investments [Member]
Defined Benefit Plan Weighted Average Asset Allocations [Abstract]
Target asset allocation percentage	15.00%
Other Postretirement Benefit Plans Defined Benefit [Member] | Business Services Company [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	24.00	[5]	20.00	[5]	14.00	[5]
Other Postretirement Benefit Plans Defined Benefit [Member] | Exelon Generation Co L L C [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	135.00	[5]	121.00	[5]	94.00	[5]
Other Postretirement Benefit Plans Defined Benefit [Member] | Commonwealth Edison Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	119.00	[5]	108.00	[5]	60.00	[5]
Other Postretirement Benefit Plans Defined Benefit [Member] | PECO Energy Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	33.00	[5]	28.00	[5]	35.00	[5]
Other Postretirement Benefit Plans Defined Benefit [Member] | Baltimore Gas and Electric Company [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	$ 12.00	[5]

[1] These amounts primarily represent amounts billed to Exelon's subsidiaries through intercompany allocations. The amounts included in capital and operating and maintenance expense for the years ended 2012, 2011, and 2010 include $12 million, $51 million, and $48 million, respectively, in costs incurred prior to the closing of Exelon’s merger with Constellation on March 12, 2012. These amounts are not included in Exelon’s capital and operating and maintenance expense for the years ended 2012, 2011, and 2010. BGE’s pension and other postretirement benefit costs for the year ended December 31, 2012 include a $3 million contractual termination benefit charge, which was recorded as a regulatory asset.

[2] Of the $1,188 million related to pension benefits, $283 million and $904 million were recognized in AOCI and regulatory assets, respectively, during 2012. Of the $120 million related to other postretirement benefits, $39 million and $81 million were recognized in AOCI and regulatory assets, respectively, during 2012. Of the $399 million related to pension benefits, $181 million and $218 million were recognized in AOCI and regulatory assets, respectively, during 2011. Of the $37 million related to other postretirement benefits, $13 million and $24 million were recognized in AOCI and regulatory assets, respectively, during 2011. Of the $464 million related to pension benefits, $310 million and $154 million were recognized in AOCI and regulatory assets, respectively, during 2010. Of the $(27) million related to other postretirement benefits, $(9) million and $(18) million were recognized in AOCI and regulatory assets, respectively, during 2010.

[3] Of the $8,007 million related to pension benefits, $4,594 million and $3,413 million are included in AOCI and regulatory assets, respectively, at December 31, 2012. Of the $1,078 million related to other postretirement benefits, $514 million and $564 million are included in AOCI and regulatory assets, respectively, at December 31, 2012. Of the $6,819 million related to pension benefits, $4,311 million and $2,508 million are included in AOCI and regulatory assets, respectively, at December 31, 2011. Of the $958 million related to other postretirement benefits, $475 million and $483 million are included in AOCI and regulatory assets, respectively, at December 31, 2011.

[4] Of the $582 million related to pension benefits at December 31, 2012, $315 million and $267 million are expected to be amortized from AOCI and regulatory assets in 2013, respectively. Of the $65 million related to other postretirement benefits at December 31, 2012, $29 million and $36 million are expected to be amortized from AOCI and regulatory assets in 2013, respectively.

[5] The Registrants present the cash contributions above net of Federal subsidy payments received on each of their respective Consolidated Statements of Cash Flows. Exelon, Generation, ComEd, PECO, and BGE received Federal subsidy payments of $10 million, $5 million, $4 million, $1 million and $2 million, respectively, in 2012, $11 million, $5 million, $4 million, $1 million and $3 million, respectively, in 2011, and $10 million, $5 million, $3 million, $2 million and $2 million, respectively, in 2010.