Retirement Benefits (Details) (USD $)	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Dec. 31, 2011
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	$ 55,000,000		$ 2,088,000,000
Defined Contribution Plan Contributions By Employer [Line Items]
Savings plan matching contributions	16,106,000		16,000,000
Fair Value Defined Benefit Plan Measured On Recurring Basis Financial Statement Captions [Line Items]
Cash equivalents	187,000,000	[1]			861,000,000	[1]
Valuation Adjustment Impact [Abstract]
Increase (decrease) in pension obligation due to updated census data	86,000,000
Increase (decrease) in other postretirement obligation due to updated census data	25,000,000
Accumulated other comprehensive loss (increase) decrease due to an updated valuation adjustment	8,000,000
Regulatory asset increase due to updated valuation adjustment	98,000,000
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	100,000,000
Expected non-qualified pension plan contributions	61,000,000
Expected other postretirement benefit plan contributions	321,000,000
Business Services Company [Member]
Defined Contribution Plan Contributions By Employer [Line Items]
Savings plan matching contributions	1,169,000	[2]	2,000,000	[2]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense [Line Items]
Amount included in capital and operating & maintenance expense	14,000,000	[2]	12,000,000	[2]
Exelon Generation Co L L C [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	20,000,000		952,000,000
Defined Contribution Plan Contributions By Employer [Line Items]
Savings plan matching contributions	7,730,000		8,000,000
Fair Value Defined Benefit Plan Measured On Recurring Basis Financial Statement Captions [Line Items]
Cash equivalents	60,000,000	[1]			466,000,000	[1]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense [Line Items]
Amount included in capital and operating & maintenance expense	81,000,000		62,000,000
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	59,000,000
Expected non-qualified pension plan contributions	9,000,000
Expected other postretirement benefit plan contributions	132,000,000
Commonwealth Edison Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	9,000,000		871,000,000
Defined Contribution Plan Contributions By Employer [Line Items]
Savings plan matching contributions	4,408,000		4,000,000
Fair Value Defined Benefit Plan Measured On Recurring Basis Financial Statement Captions [Line Items]
Cash equivalents	3,000,000	[1]			173,000,000	[1]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense [Line Items]
Amount included in capital and operating & maintenance expense	69,000,000		54,000,000
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	12,000,000
Expected non-qualified pension plan contributions	14,000,000
Expected other postretirement benefit plan contributions	116,000,000
PECO Energy Co [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	5,000,000		110,000,000
Defined Contribution Plan Contributions By Employer [Line Items]
Savings plan matching contributions	1,799,000		2,000,000
Fair Value Defined Benefit Plan Measured On Recurring Basis Financial Statement Captions [Line Items]
Cash equivalents	53,000,000				175,000,000	[1]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense [Line Items]
Amount included in capital and operating & maintenance expense	13,000,000		8,000,000
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	16,000,000
Expected non-qualified pension plan contributions	1,000,000
Expected other postretirement benefit plan contributions	34,000,000
Baltimore Gas and Electric Company [Member]
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions	7,000,000		5,000,000
Defined Contribution Plan Contributions By Employer [Line Items]
Savings plan matching contributions	2,000,000	[3]	2,000,000	[3]
Pension And Other Postretirement Benefit Expense Included In Capital And Operating And Maintenance Expense [Line Items]
Amount included in capital and operating & maintenance expense	16,000,000	[4]	11,000,000	[4]
Defined Benefit Plan Contributions [Abstract]
Expected non-qualified pension plan contributions	1,000,000
Expected other postretirement benefit plan contributions	17,000,000
Expected other postretirement benefit plan contributions for acquired entities	4,000,000
Pension Plans Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost [Abstract]
Expected return on plan assets	7.50%
Discount rate	4.74%
Discount rate from acquisition entity	4.27%
Components of net periodic benefit cost:
Service cost	61,000,000		53,000,000
Interest cost	164,000,000		162,000,000
Expected return on assets	(232,000,000)		(235,000,000)
Amortization of:
Transition obligation	0		0
Prior service cost (credit)	4,000,000		4,000,000
Actuarial loss	106,000,000		83,000,000
Net periodic benefit cost	103,000,000		67,000,000
Change in benefit obligation:
Service cost	61,000,000		53,000,000
Interest cost	164,000,000		162,000,000
Other Postretirement Benefit Plans Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost [Abstract]
Expected return on plan assets	6.68%
Discount rate	4.80%
Discount rate from acquisition entity	4.28%
Components of net periodic benefit cost:
Service cost	37,000,000		36,000,000
Interest cost	51,000,000		52,000,000
Expected return on assets	(29,000,000)		(28,000,000)
Amortization of:
Transition obligation	3,000,000		2,000,000
Prior service cost (credit)	(3,000,000)		(9,000,000)
Actuarial loss	19,000,000		16,000,000
Net periodic benefit cost	78,000,000		69,000,000
Change in benefit obligation:
Service cost	37,000,000		36,000,000
Interest cost	$ 51,000,000		$ 52,000,000

[1] Excludes certain cash equivalents considered to be held-to-maturity and not reported at fair value.

[2] BGE’s pension and postretirement benefit costs for the three months ended March 31, 2012 includes $million of cost incurred prior to the closing of the merger with Constellation on March 12, 2012. This amount is not included Exelon’s net periodic benefit for the three months ended March 31, 2012 shown in the table above. (b)        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.

[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.

[4] BGE's pension and postretirement benefit costs for the three months ended March 31, 2012 and 2011 include $12 million and $11 million, respectively, 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 cost for the three months ended March 31, 2012 and 2011 shown in the first table of the Defined Benefit Pension and Other Postretirement Benefits section above.