Retirement Benefits (Details) (USD $)	3 Months Ended			9 Months Ended		3 Months Ended						9 Months Ended				3 Months Ended			9 Months Ended		3 Months Ended			9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended						9 Months Ended				3 Months Ended		9 Months Ended			3 Months Ended				9 Months Ended			3 Months Ended			3 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2012 Exelon Legacy Benefit Plans [Member]	Jun. 30, 2012 Constellation Legacy Benefit Plans [Member]	Sep. 30, 2012 Business Services Company [Member]		Sep. 30, 2011 Business Services Company [Member]		Sep. 30, 2012 Business Services Company [Member]		Sep. 30, 2011 Business Services Company [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member]	Jun. 30, 2012 Exelon Generation Co L L C [Member]	Sep. 30, 2011 Exelon Generation Co L L C [Member]	Sep. 30, 2012 Exelon Generation Co L L C [Member]	Sep. 30, 2011 Exelon Generation Co L L C [Member]	Sep. 30, 2012 Commonwealth Edison Co [Member]	Jun. 30, 2012 Commonwealth Edison Co [Member]	Sep. 30, 2011 Commonwealth Edison Co [Member]	Sep. 30, 2012 Commonwealth Edison Co [Member]	Sep. 30, 2011 Commonwealth Edison Co [Member]	Sep. 30, 2012 PECO Energy Co [Member]	Sep. 30, 2011 PECO Energy Co [Member]	Sep. 30, 2012 PECO Energy Co [Member]	Sep. 30, 2011 PECO Energy Co [Member]	Mar. 11, 2012 Baltimore Gas and Electric Company [Member]	Sep. 30, 2012 Baltimore Gas and Electric Company [Member]		Jun. 30, 2012 Baltimore Gas and Electric Company [Member]	Sep. 30, 2011 Baltimore Gas and Electric Company [Member]		Sep. 30, 2012 Baltimore Gas and Electric Company [Member]		Sep. 30, 2011 Baltimore Gas and Electric Company [Member]		Sep. 30, 2012 Pension Plans Defined Benefit [Member]	Sep. 30, 2011 Pension Plans Defined Benefit [Member]	Sep. 30, 2012 Pension Plans Defined Benefit [Member]		Sep. 30, 2011 Pension Plans Defined Benefit [Member]	Mar. 31, 2012 Pension Plans Defined Benefit [Member] Exelon Legacy Benefit Plans [Member]	Jun. 30, 2012 Pension Plans Defined Benefit [Member] Constellation Legacy Benefit Plans [Member]	Sep. 30, 2012 Other Postretirement Benefit Plans Defined Benefit [Member]	Sep. 30, 2011 Other Postretirement Benefit Plans Defined Benefit [Member]	Sep. 30, 2012 Other Postretirement Benefit Plans Defined Benefit [Member]		Sep. 30, 2011 Other Postretirement Benefit Plans Defined Benefit [Member]	Mar. 31, 2012 Other Postretirement Benefit Plans Defined Benefit [Member] Exelon Legacy Benefit Plans [Member]	Jun. 30, 2012 Other Postretirement Benefit Plans Defined Benefit [Member] Constellation Legacy Benefit Plans [Member]	Sep. 30, 2012 Other Postretirement Benefit Plans Defined Benefit [Member] Exelon Generation Co L L C [Member]	Jun. 30, 2012 Other Postretirement Benefit Plans Defined Benefit [Member] Exelon Generation Co L L C [Member] Constellation Legacy Benefit Plans [Member]	Sep. 30, 2012 Other Postretirement Benefit Plans Defined Benefit [Member] Baltimore Gas and Electric Company [Member]
Change in benefit obligation:
Service cost																																								$ 76,000,000	$ 53,000,000	$ 211,000,000		$ 159,000,000			$ 38,000,000	$ 36,000,000	$ 114,000,000		$ 107,000,000
Interest cost																																								181,000,000	162,000,000	524,000,000		487,000,000			53,000,000	52,000,000	157,000,000		155,000,000
Actuarial loss (gain)																																								117,000,000	83,000,000	338,000,000		248,000,000			19,000,000	16,000,000	58,000,000		49,000,000
Curtailments/settlements																																																						1,000,000		3,000,000
Components of net periodic benefit cost:
Service cost																																								76,000,000	53,000,000	211,000,000		159,000,000			38,000,000	36,000,000	114,000,000		107,000,000
Interest cost																																								181,000,000	162,000,000	524,000,000		487,000,000			53,000,000	52,000,000	157,000,000		155,000,000
Expected return on assets																																								(258,000,000)	(235,000,000)	(742,000,000)		(704,000,000)			(28,000,000)	(27,000,000)	(86,000,000)		(83,000,000)
Amortization of:
Transition obligation																																															2,000,000	2,000,000	8,000,000		7,000,000
Prior service cost (credit)																																								5,000,000	4,000,000	12,000,000		11,000,000			(3,000,000)	(10,000,000)	(10,000,000)		(29,000,000)
Actuarial loss (gain)																																								117,000,000	83,000,000	338,000,000		248,000,000			19,000,000	16,000,000	58,000,000		49,000,000
Curtailments/settlements charges																																								9,000,000		9,000,000					(5,000,000)		(7,000,000)						1,000,000
Contractual termination benefit cost		20,000,000															9,000,000																3,000,000									14,000,000	[1]						6,000,000	[1]
Net periodic benefit cost																																								130,000,000	67,000,000	366,000,000		201,000,000			76,000,000	69,000,000	240,000,000		206,000,000
Pension And Other Postretirement Benefit Contributions [Abstract]
Pension and non-pension postretirement benefit contributions				131,000,000	2,089,000,000														48,000,000	952,000,000				19,000,000	871,000,000			12,000,000	110,000,000							13,000,000		17,000,000
Defined Benefit Plan Expense Included In Capital And Operational And Maintenance Expense [Abstract]
Amount included in capital and operating & maintenance expense								20,000,000	[2]	11,000,000	[2]	63,000,000	[2]	36,000,000	[2]	85,000,000		64,000,000	259,000,000	187,000,000	75,000,000		53,000,000	212,000,000	160,000,000	12,000,000	8,000,000	38,000,000	24,000,000	12,000,000	14,000,000	[3]		15,000,000	[3]	46,000,000	[3],[4]	43,000,000	[3],[4]
Defined Contribution Plan Contributions By Employer [Abstract]
Savings plan matching contributions	16,000,000		26,000,000	49,000,000	64,000,000			1,000,000		2,000,000		3,000,000		6,000,000		7,000,000		13,000,000	23,000,000	33,000,000	5,000,000		8,000,000	14,000,000	18,000,000	2,000,000	3,000,000	5,000,000	7,000,000	1,000,000	1,000,000	[5]		1,000,000	[5]	5,000,000	[5]	5,000,000	[5]
Valuation Adjustment Impact [Abstract]
Benefit obligation increase (decrease) reflecting actual census data																																													86,000,000	1,000,000						25,000,000	(19,000,000)
Benefit obligation increase (decrease) due to curtailment																																																					(32,000,000)
Other comprehensive income (loss) due to an updated valuation adjustment						(8,000,000)	3,000,000
Other comprehensive income (loss) due to curtailment							(6,000,000)																																							84,000,000
Regulatory asset increase (decrease) due to updated valuation adjustment						98,000,000	(13,000,000)
Regulatory asset increase (decrease) due to curtailment							44,000,000
Regulatory asset increase (decrease) due to contractual termination benefits																																	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
Discount rate for curtailed plans																																														3.97%							3.98%
Design Changes Impact [Abstract]
Benefit Obligation increase (decrease) due to design changes																																															212,000,000
Other comprehensive income (loss) due to design changes																																															(68,000,000)
Regulatory Asset increase (decrease) due to design changes																																															99,000,000
Settlement Charges [Abstract]
Future settlement charge to be recorded				$ 17,000,000

[1] 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.

[2] 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.

[3] BGE's pension and other postretirement benefit costs for the nine months ended September 30, 2012 includes a $3 million contractual termination benefit charge, which was recorded as a regulatory asset as of September 30, 2012.

[4] 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. BGE's pension and postretirement benefit costs for the three months and nine months ended September 30, 2011 were $15 million and $43 million, respectively. These amounts are not included in Exelon's net periodic benefit costs for the three and nine months ended September 30, 2012 and 2011 shown in the first table of the Defined Benefit Pension and Other Postretirement Benefits section above.

[5] 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.