Retirement Benefits (USD $)	3 Months Ended					6 Months Ended
	Mar. 11, 2012	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Change in benefit obligation:
Special termination benefits		$ 20,000,000				$ 20,000,000
Amortization of:
Curtailments/settlements charges		0
Special termination benefits		(20,000,000)				(20,000,000)
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions						90,000,000		2,089,000,000
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions		17,000,000		15,000,000		33,000,000		34,000,000
Exelon Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Other comprehensive income (loss) due to an updated valuation adjustment						(8,000,000)
Regulatory asset increase (decrease) due to updated valuation adjustment						98,000,000
Constellation Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Other comprehensive income (loss) due to an updated valuation adjustment		3,000,000				3,000,000
Other comprehensive income (loss) due to curtailment		(6,000,000)				(6,000,000)
Regulatory asset increase (decrease) due to updated valuation adjustment		(13,000,000)				(13,000,000)
Regulatory asset increase (decrease) due to curtailment		44,000,000				44,000,000
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		25,000,000	[1]	12,000,000	[1]	42,000,000	[1]	24,000,000	[1]
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions		1,000,000		1,000,000		2,000,000		2,000,000
Exelon Generation Co L L C [Member]
Change in benefit obligation:
Special termination benefits		9,000,000				9,000,000
Amortization of:
Special termination benefits		(9,000,000)				(9,000,000)
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions						35,000,000		952,000,000
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense		94,000,000		61,000,000		175,000,000		123,000,000
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions		8,000,000		8,000,000		15,000,000		18,000,000
Commonwealth Edison Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions						12,000,000		871,000,000
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense		68,000,000		54,000,000		137,000,000		108,000,000
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions		5,000,000		4,000,000		9,000,000		10,000,000
Valuation Adjustment Impact [Abstract]
Regulatory asset increase (decrease) due to contractual termination benefit billed from affiliate		1,000,000				1,000,000
PECO Energy Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions						8,000,000		110,000,000
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense		13,000,000		8,000,000		26,000,000		16,000,000
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions		2,000,000		2,000,000		4,000,000		4,000,000
Baltimore Gas and Electric Company [Member]
Change in benefit obligation:
Special termination benefits		3,000,000				3,000,000
Amortization of:
Special termination benefits		(3,000,000)				(3,000,000)
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions						8,000,000		7,000,000
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense	12,000,000	18,000,000	[2]	14,000,000	[2]	32,000,000	[2],[3]	28,000,000	[2],[3]
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	1,000,000	2,000,000	[4]	2,000,000	[4]	4,000,000	[4]	4,000,000	[4]
Valuation Adjustment Impact [Abstract]
Regulatory asset increase (decrease) due to contractual termination benefits		3,000,000				3,000,000
Regulatory asset increase (decrease) due to contractual termination benefit billed from affiliate		1,000,000				1,000,000
Total regulatory asset increase (decrease) due to due contractual termination benefit		4,000,000				4,000,000
Pension Plans Defined Benefit [Member]
Change in benefit obligation:
Service cost		74,000,000		53,000,000		135,000,000		106,000,000
Interest cost		179,000,000		163,000,000		343,000,000		325,000,000
Special termination benefits		(14,000,000)	[5]	0	[5]	(14,000,000)	[5]	0	[5]
Components of net periodic benefit cost:
Service cost		74,000,000		53,000,000		135,000,000		106,000,000
Interest cost		179,000,000		163,000,000		343,000,000		325,000,000
Expected return on assets		(253,000,000)		(234,000,000)		(484,000,000)		(469,000,000)
Amortization of:
Transition obligation		0		0		0		0
Prior service cost (credit)		3,000,000		3,000,000		7,000,000		7,000,000
Actuarial loss		115,000,000		82,000,000		221,000,000		165,000,000
Curtailments/settlements charges				0		0		0
Special termination benefits		14,000,000	[5]	0	[5]	14,000,000	[5]	0	[5]
Net periodic benefit cost		132,000,000		67,000,000		236,000,000		134,000,000
Pension Plans Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Benefit obligation increase (decrease) reflecting actual census data						86,000,000
Pension Plans Defined Benefit [Member] | Constellation Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Benefit obligation increase (decrease) reflecting actual census data		1,000,000				1,000,000
Other comprehensive income (loss) due to curtailment		84,000,000				84,000,000
Discount rate for curtailed plans						3.97%
Other Postretirement Benefit Plans Defined Benefit [Member]
Change in benefit obligation:
Service cost		39,000,000		35,000,000		76,000,000		71,000,000
Interest cost		54,000,000		51,000,000		104,000,000		103,000,000
Special termination benefits		(6,000,000)	[5]	0	[5]	(6,000,000)	[5]	0	[5]
Components of net periodic benefit cost:
Service cost		39,000,000		35,000,000		76,000,000		71,000,000
Interest cost		54,000,000		51,000,000		104,000,000		103,000,000
Expected return on assets		(29,000,000)		(28,000,000)		(58,000,000)		(56,000,000)
Amortization of:
Transition obligation		2,000,000		3,000,000		6,000,000		5,000,000
Prior service cost (credit)		(3,000,000)		(10,000,000)		(6,000,000)		(19,000,000)
Actuarial loss		19,000,000		17,000,000		38,000,000		33,000,000
Curtailments/settlements charges		(2,000,000)		0		(2,000,000)		0
Special termination benefits		6,000,000	[5]	0	[5]	6,000,000	[5]	0	[5]
Net periodic benefit cost		86,000,000		68,000,000		164,000,000		137,000,000
Other Postretirement Benefit Plans Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Benefit obligation increase (decrease) reflecting actual census data						25,000,000
Other Postretirement Benefit Plans Defined Benefit [Member] | Constellation Legacy Benefit Plans [Member]
Valuation Adjustment Impact [Abstract]
Benefit obligation increase (decrease) reflecting actual census data		(19,000,000)				(19,000,000)
Benefit obligation increase (decrease) due to curtailment		(32,000,000)				(32,000,000)
Discount rate for curtailed plans						3.98%
Other Postretirement Benefit Plans Defined Benefit [Member] | Exelon Generation Co L L C [Member] | Constellation Legacy Benefit Plans [Member]
Amortization of:
Curtailments/settlements charges		$ 1,000,000				$ 1,000,000

[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 other postretirement benefit costs for the three and six months ended June 30, 2012 includes a $3 million contractual termination benefit charge, which was recorded as a regulatory asset as of June 30, 2012.

[3] BGE's pension and postretirement benefit costs for the six months ended June 30, 2012 and 2011 include $12 million and $28 million, respectively, of costs incurred prior to the closing of Exelon's merger with Constellation on March 12, 2012. BGE's pension and postretirement benefit costs for the three months ended June 30, 2011 include $14 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 and six months ended June 30, 2012 and 2011 shown in the first table of the Defined Benefit Pension and Other Postretirement Benefits section above.

[4] 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.

[5] As discussed above, 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.