Retirement Benefits Retirement Benefits- Narrative (Details) - USD ($) $ in Millions			3 Months Ended			9 Months Ended		12 Months Ended
		Jan. 01, 2016	Sep. 30, 2017	Mar. 31, 2017	Mar. 23, 2016	Dec. 31, 2016		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount	[1]								$ 128		$ 164		$ 148
Increase (Decrease) in Obligation, Pension Benefits				$ 92
Benefit obligation increase (decrease) reflecting actual census data				(57)
AOCI valuation adjustment				(59)
Regulatory asset increase (decrease) due to updated valuation adjustment				57
Defined benefit plan, amounts recognized In regulatory liabilities, before tax				$ 4
Expected qualified pension plan contributions	[2]								301
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[3]								30
Expected other postretirement benefit plan contributions	[4]								$ 42
Fitzpatrick [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Increase (Decrease) in Obligation, Pension Benefits			$ 16
Benefit obligation increase (decrease) reflecting actual census data			$ (17)
Projected Benefit Obligation In Excess Of Plan Assets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Funded Percentage						80.00%			83.00%		80.00%
Accumulated Benefit Obligation In Excess Of Plan Assets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Funded Percentage						84.00%			88.00%		84.00%
Exelon Generation Co L L C [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount	[1]								$ 55		$ 79		80
Expected qualified pension plan contributions	[2]								119
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[3]								11
Expected other postretirement benefit plan contributions	[4]								13
Commonwealth Edison Co [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount									31		34		32
Expected qualified pension plan contributions	[2]								38
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[3]								2
Expected other postretirement benefit plan contributions	[4]								3
PECO Energy Co [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount									10		10		11
Expected qualified pension plan contributions	[2]								17
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[3]								1
Expected other postretirement benefit plan contributions	[4]								0
Baltimore Gas and Electric Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount									10		12		14
Expected qualified pension plan contributions	[2]								41
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[3]								1
Expected other postretirement benefit plan contributions	[4]								16
Pepco Holdings LLC [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected qualified pension plan contributions	[2]								50
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[3]								8
Expected other postretirement benefit plan contributions	[4]								9
Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount					$ 3						14
Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount						$ 10			13
Potomac Electric Power Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount					1				3	[5]	3	[5]	3	[5]
Expected qualified pension plan contributions	[2]								4
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[3]								2
Expected other postretirement benefit plan contributions	[4]								8
Delmarva Power and Light Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount					1				2	[5]	2	[5]	2	[5]
Expected qualified pension plan contributions	[2]								0
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[3]								1
Expected other postretirement benefit plan contributions	[4]								0
Atlantic City Electric Company [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Discretionary Contribution Amount	[5]								2		2		2
Expected qualified pension plan contributions	[2]								6
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[3]								0
Expected other postretirement benefit plan contributions	[4]								0
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)									607	[6]	554	[7]	$ 571
Actual return on plan assets									2,600		$ 1,061	[8]
Defined Contribution Plan, Employer Discretionary Contribution Amount									$ 300
Discount rate						4.04%	[9]		3.62%	[10]	4.04%	[9]	4.29%	[11]
Current year actuarial (gain) loss									$ (222)		$ 644	[12]	$ 476
Regulatory asset increase (decrease) due to updated valuation adjustment									$ (423)		$ 959	[12]	$ (46)
Expected return on plan assets								7.00%	7.00%		7.00%	[13]	7.00%	[13]
Discount rate									4.04%	[14]	4.29%	[15]	3.94%	[16]
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months									$ 1,166
Amortization of actuarial loss									(607)		$ (554)	[12]	$ (571)
Current year prior service cost (credit)									9		(60)	[12]	0
Amortization of prior service (cost) credit									(1)		(14)	[12]	(13)
Settlements									(3)		0	[12]	(2)
Defined Benefit Plan Accumulated Other Comprehensive Income And Regulatory Assets Before Tax	[12]								(824)		1,010		(110)
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)					(14)						(65)
Current year actuarial (gain) loss					0						50
Regulatory asset increase (decrease) due to updated valuation adjustment					$ (13)						$ (6)
Expected return on plan assets	[17]				6.50%						6.50%
Discount rate											4.20%
Amortization of actuarial loss					$ (14)						$ (65)
Amortization of prior service (cost) credit					0						(2)
Defined Benefit Plan Accumulated Other Comprehensive Income And Regulatory Assets Before Tax	[12]				$ (14)						$ (17)
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase					5.00%						5.00%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)									61	[6]	$ 63	[7]	$ 80
Actual return on plan assets									$ 346		$ 128	[8]
Discount rate						4.04%	[9]		3.61%	[10]	4.04%	[9]	4.29%	[11]
Current year actuarial (gain) loss									$ 166		$ (101)	[12]	$ (194)
Regulatory asset increase (decrease) due to updated valuation adjustment									$ 125		$ 95	[12]	$ (60)
Expected return on plan assets								6.60%	6.58%		6.71%	[13]	6.50%	[13]
Discount rate									4.04%	[14]	4.29%	[15]	3.92%	[16]
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months									$ 256
Amortization of actuarial loss									(61)		$ (63)	[12]	$ (80)
Current year prior service cost (credit)									0		0	[12]	(23)
Amortization of prior service (cost) credit									188		185	[12]	174
Settlements									0		0	[12]	0
Defined Benefit Plan Accumulated Other Comprehensive Income And Regulatory Assets Before Tax	[12]								$ 293		115		$ (123)
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Amortization of Gain (Loss)					$ (2)						$ (8)
Discount rate						4.55%					4.55%
Current year actuarial (gain) loss					0						$ (39)
Regulatory asset increase (decrease) due to updated valuation adjustment					$ 1						$ (34)
Expected return on plan assets	[17]				6.75%						6.75%
Discount rate					4.55%						4.15%
Amortization of actuarial loss					$ (2)						$ (8)
Amortization of prior service (cost) credit					3						13
Defined Benefit Plan Accumulated Other Comprehensive Income And Regulatory Assets Before Tax	[12]				$ 1						$ (34)
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase					5.00%						5.00%
Change in Assumptions for Defined Benefit Plans [Member] | 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%
Discount rate									4.04%
Change in Assumptions for Defined Benefit Plans [Member] | 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		6.58%
Discount rate									4.04%
Assets [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate									16.10%
Assets [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate									14.70%

[1] Includes $13 million and $9 million related to CENG for the years ended December 31, 2016 and December 31, 2015.

[2] Qualified Pension PlansNon-Qualified Pension PlansOtherPostretirementBenefitsExelon$301$30$42Generation1191113ComEd3823PECO171—BGE41116BSC3671PHI5089Pepco428DPL—1—ACE6——PHISCO4051

[3] Qualified Pension PlansNon-Qualified Pension PlansOtherPostretirementBenefitsExelon$301$30$42Generation1191113ComEd3823PECO171—BGE41116BSC3671PHI5089Pepco428DPL—1—ACE6——PHISCO4051

[4] Qualified Pension PlansNon-Qualified Pension PlansOtherPostretirementBenefitsExelon$301$30$42Generation1191113ComEd3823PECO171—BGE41116BSC3671PHI5089Pepco428DPL—1—ACE6——PHISCO4051

[5] Pepco's, DPL's and PHISCO's matching contributions include $1 million, $1 million and $1 million, respectively, of costs incurred prior to the closing of Exelon's merger with PHI on March 23, 2016, which is not included in Exelon's matching contributions for the year ended December 31, 2016.

[6] FitzPatrick net benefit costs are included for the period after acquisition.

[7] PHI net periodic benefit costs for the period prior to the merger are not included in the table above.

[8] 2016 amounts include PHI for the period of March 24, 2016 through December 31, 2016.

[9] The discount rates above represent the blended rates used to determine the majority of Exelon’s pension and other postretirement benefits obligations as of December 31, 2016. Certain benefit plans used individual rates ranging from 3.66% - 4.11% and 4.00% - 4.17% for pension and other postretirement plans, respectively.

[10] The discount rates above represent the blended rates used to determine the majority of Exelon’s pension and other postretirement benefits obligations as of December 31, 2017. Certain benefit plans used individual rates ranging from 3.49% - 3.65% and 3.57% - 3.68% for pension and other postretirement plans, respectively.

[11] The discount rates above represent the blended rates used to determine the majority of Exelon’s pension and other postretirement benefits obligations as of December 31, 2015. Certain benefit plans used individual rates ranging from 3.68% - 4.14% and 4.32% - 4.43% for pension and other postretirement plans, respectively.

[12] 2016 amounts include PHI for the period of March 24, 2016 through December 31, 2016.

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

[14] )The discount rates above represent the blended rates used to establish the majority of Exelon’s pension and other postretirement benefits costs for the year ended December 31, 2017. Certain benefit plans used individual rates ranging from 3.66%-4.11% and 4.00%-4.17% for pension and other postretirement plans, respectivel

[15] The discount rates above represent the blended rates used to establish the majority of Exelon's pension and other postretirement benefits costs for the year ended December 31, 2016. Certain benefit plans used individual rates ranging from 3.68%-4.14% and 4.32%-4.43% for pension and other postretirement plans, respectively.

[16] The discount rates above represent the blended rates used to establish the majority of Exelon’s pension and other postretirement benefits costs for the year ended December 31, 2015. Certain benefit plans used the individual rates ranging from 3.29%-3.82% and 3.99%-4.06% for pension and other postretirement plans, respectively.

[17] Expected return on other postretirement benefit plan assets is pre-tax.