Retirement Benefits - Contributions made to Pension and Other Postretirement Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	$ 617		$ 422		$ 462
Expected qualified pension plan contributions	447
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	332	[1]	339		149
Service cost	293		317		280
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received					10
Pension and non-pension postretirement benefit contributions	291		83	[2]	323	[2]
Service cost	117		162		156
Defined Contribution Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	250
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	297		149		178
Expected qualified pension plan contributions	230
Exelon Generation Co L L C [Member] | CENG [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected non-qualified pension plan contributions	4
Exelon Generation Co L L C [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	173	[1]	119		48
Exelon Generation Co L L C [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received					5
Pension and non-pension postretirement benefit contributions	124		30	[2]	135	[2]
Exelon Corporate [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	9	[1]	72		13
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	248		122		138
Expected qualified pension plan contributions	138
Commonwealth Edison Co [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	122	[1]	118		25
Commonwealth Edison Co [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received					4
Pension and non-pension postretirement benefit contributions	125		4	[2]	119	[2]
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	16		31		45
Expected qualified pension plan contributions	40
PECO Energy Co [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	11	[1]	11		13
PECO Energy Co [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received					1
Pension and non-pension postretirement benefit contributions	5		20	[2]	33	[2]
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	16		24		16
Expected qualified pension plan contributions	1
Baltimore Gas and Electric Company [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	0	[1],[3]	0	[3]	0	[3]
Baltimore Gas and Electric Company [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received					2
Pension and non-pension postretirement benefit contributions	17	[3]	24	[2],[3]	12	[2],[3]
Business Services Company [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	26	[1]	91		63
Business Services Company [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	20		5	[2]	24	[2]
Constellation Legacy Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	$ 43	[1]

[1] )Exelon's and Generation's pension contributions include $43 million related to the legacy CENG plans that was funded by CENG as provided in an Employee Matters Agreement (EMA) between Exelon and CEN

[2] )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. Effective January 1, 2013, Exelon is no longer receiving this subsid

[3] )BGE’s other postretirement benefit payments for 2012 exclude $4 million, of other postretirement benefit payments made by BGE prior to the closing of the Constellation merger on March 12, 2012. These pre-Constellation merger contributions are not included in Exelon’s financial statements but are reflected in BGE’s financial statement