Retirement Benefits - Narrative (Details) (USD $)	3 Months Ended	12 Months Ended						1 Months Ended	3 Months Ended	0 Months Ended
	Mar. 31, 2014	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Apr. 30, 2014	Jun. 30, 2014	Dec. 31, 2014	Oct. 31, 2014	Apr. 30, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, amounts recognized In regulatory liabilities, before tax	$ 5,000,000
Effect of federal subsidy on net periodic postretirement benefit costs under the Prescription Drug Act		0		0		17,000,000
Expected qualified pension plan contributions		447,000,000								447,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months		37,000,000								37,000,000
Projected Benefit Obligation In Excess Of Plan Assets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Funded status of the pension and other postretirement benefit obligations		81.00%		88.00%
Accumulated Benefit Obligation In Excess Of Plan Assets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Funded status of the pension and other postretirement benefit obligations		87.00%		93.00%
Exelon Generation Co L L C [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected qualified pension plan contributions		230,000,000								230,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months		17,000,000								17,000,000
Exelon Generation Co L L C [Member] | CENG [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected non-qualified pension plan contributions		4,000,000								4,000,000
Business Services Company [Member] | Remeasurement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset increase (decrease) due to updated valuation adjustment								125,000,000
Commonwealth Edison Co [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected qualified pension plan contributions		138,000,000								138,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months		1,000,000								1,000,000
Expected other postretirement benefit plan contributions		2,000,000								2,000,000
PECO Energy Co [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected qualified pension plan contributions		40,000,000								40,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months		1,000,000								1,000,000
Expected other postretirement benefit plan contributions		0								0
Baltimore Gas and Electric Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected qualified pension plan contributions		1,000,000								1,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months		1,000,000								1,000,000
Expected other postretirement benefit plan contributions		17,000,000								17,000,000
Exelon Legacy Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset increase (decrease) due to updated valuation adjustment	34,000,000
Constellation Legacy Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
AOCI valuation adjustment	12,000,000								1,000,000
Regulatory asset increase (decrease) due to updated valuation adjustment									15,000,000
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets		7.00%	[1]	7.50%	[1]	7.50%	[1]				7.00%
Discount rate		4.80%	[2]	3.92%	[3]	4.74%	[4]				3.95%
Increase (decrease) in pension obligation										361,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months		1,064,000,000								1,064,000,000
Healthcare cost trend on covered charges		7.00%
Pension Plan, Defined Benefit [Member] | Remeasurement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets								6.59%
Pension Plan, Defined Benefit [Member] | Exelon Generation Co L L C [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months		6,000,000								6,000,000
Pension Plan, Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefit obligation increase (decrease) reflecting actual census data	35,000,000								13,000,000
Pension Plan, Defined Benefit [Member] | Constellation Legacy Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets		7.75%
Pension Plan, Defined Benefit [Member] | Constellation Legacy Benefit Plans [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate		3.60%
Pension Plan, Defined Benefit [Member] | Constellation Legacy Benefit Plans [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate		4.30%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefit obligation increase (decrease) due to design changes								790,000,000
Regulatory asset increase (decrease) due to design changes								240,000,000
Other comprehensive income (loss) due to design changes								259,000,000
Expected return on plan assets		6.59%	[1]	6.45%	[1]	6.68%	[1]					6.59%
Discount rate		4.90%	[2]	4.00%	[3]	4.80%	[4]					4.30%
Increase (decrease) in pension obligation										117,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months		217,000,000								217,000,000
Healthcare cost trend on covered charges		6.46%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefit obligation increase (decrease) reflecting actual census data	12,000,000								3,000,000
Other Postretirement Benefit Plan, Defined Benefit [Member] | Constellation Legacy Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets		4.55%
Exelon Sponsored Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected qualified pension plan contributions		36,000,000								36,000,000
Non-Qualified Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months		$ 15,000,000								$ 15,000,000

[1] Not applicable to pension and other postretirement benefit plans that do not have plan asset

[2] )The discount rates above represent the initial discount rates used to establish the majority of Exelon’s pension and other postretirement benefits costs for the year ended December 31, 2014. Certain of the other postretirement benefit plans were remeasured as of April 30, 2014 using an expected long-term rate of return on plan assets of 6.59% and a discount rate of 4.30%. Costs for the year ended December 31, 2014 reflect the impact of this remeasurement. On April 1, 2014, Generation assumed operational control of CENG’s nuclear fleet. As a result, Exelon became the sponsor of CENG’s legacy pension and OPEB plans effective July 14, 2014; discount rates for those plans, impacting 2014 costs, ranged from 3.60%-4.30% and 4.09%-4.55%, respectivel

[3] The discount rates above represent the initial discount rates used to establish Exelon's pension and other postretirement benefits costs for the year ended December 31, 2013. Certain of the benefit plans were remeasured during the year using discount rates of 4.21% and 4.66% for pension and other postretirement benefits, respectively. Costs for the year ended December 31, 2013 reflect the impact of these measurements

[4] The discount rates above represent the initial discounts rates used to establish Exelon’s pension and other postretirement benefits costs for the year ended December 31, 2012. Certain of the benefit plans were remeasured during the year due to the Constellation merger, plan settlement and curtailment events, and plan changes using discount rates of 3.71% and 3.72% for pension and other postretirement benefits, respectively. Costs for the year ended December 31, 2012 reflect the impact of these remeasurement