Retirement Benefits - Narrative (Details) - USD ($)			1 Months Ended	3 Months Ended	12 Months Ended
		Dec. 31, 2014	Apr. 30, 2014	Mar. 31, 2015	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, amounts recognized In regulatory liabilities, before tax				$ 11,000,000
Effect of federal subsidy on net periodic postretirement benefit costs under the Prescription Drug Act					$ 0		$ 0
Expected qualified pension plan contributions					250,000,000
Expected other postretirement benefit plan contributions					$ 35,000,000
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%		81.00%
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					85.00%		87.00%
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					$ 134,000,000
Expected other postretirement benefit plan contributions					13,000,000
Commonwealth Edison Co [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected qualified pension plan contributions					30,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months					2,000,000
Expected other postretirement benefit plan contributions					3,000,000
PECO Energy Co [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected qualified pension plan contributions					28,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months					1,000,000
Expected other postretirement benefit plan contributions					1,000,000
Baltimore Gas and Electric Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected qualified pension plan contributions					31,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months					1,000,000
Expected other postretirement benefit plan contributions					$ 18,000,000
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				48,000,000
Constellation Legacy Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
AOCI valuation adjustment				27,000,000
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets	[1]				7.00%		7.00%		7.50%
Discount rate					3.94%	[2]	4.80%	[3]	3.92%	[4]
Increase (decrease) in pension obligation		$ 361,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months					$ 1,153,000,000
Healthcare cost trend on covered charges					7.00%
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					$ 9,000,000
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				45,000,000
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets			6.46%		6.50%	[1]	6.59%	[1]	6.45%	[1]
Discount rate					3.92%	[2]	4.90%	[3]	4.00%	[4]
Increase (decrease) in pension obligation		$ 117,000,000
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months					$ 217,000,000
Healthcare cost trend on covered charges					6.71%
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				$ 57,000,000
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					$ 21,000,000

[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, 2015. Discount rates for CENG's legacy pension and OPEB plans ranged from 3.68%-4.14% and 4.32%-4.43%, respectivel

[3] 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 of 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%, respectively. See Note 5 - Investment in Constellation Energy Nuclear Group, LLC for further information.

[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, 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 remeasurement