Retirement Benefits Retirement Benefits - Summary of Changes in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 23, 2016	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Employer contributions					$ 13
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement			$ 48		(34)
Net benefit obligation at beginning of year			22,337		21,060
Service cost			405	[1]	387	[2]	$ 354
Interest cost			802	[1]	842	[2]	830
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant			0		0
Actuarial (gain) loss(a)	[3]		(1,561)		1,182
Plan amendments			(4)		9
Acquisitions(b)	[4]		0		16
Defined Benefit Plan, Benefit Obligation, Benefits Paid			(1,239)		(1,125)
Net benefit obligation at end of year			20,692		22,337		21,060
Fair value of net plan assets at beginning of year			18,573		16,791
Actual return on plan assets			(945)		2,600
Employer contributions			337		341
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant			0		0
Defined Benefit Plan, Plan Assets, Benefits Paid			(1,239)		(1,125)
Defined Benefit Plan, Fair Value of Plan Assets			16,678		18,573		16,791
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost		$ 12
Interest cost		26
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement			4		0
Net benefit obligation at beginning of year			4,856		4,457
Service cost			112	[1]	106	[2]	107
Interest cost			175	[1]	182	[2]	185
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant			45		53
Actuarial (gain) loss(a)	[3]		(540)		350
Plan amendments			0		0
Acquisitions(b)	[4]		0		17
Defined Benefit Plan, Benefit Obligation, Benefits Paid			(275)		(309)
Net benefit obligation at end of year			4,369		4,856		4,457
Fair value of net plan assets at beginning of year			2,732		2,578
Actual return on plan assets			(136)		346
Employer contributions			46		64
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant			45		53
Defined Benefit Plan, Plan Assets, Benefits Paid			(275)		(309)
Defined Benefit Plan, Fair Value of Plan Assets			$ 2,408		$ 2,732		$ 2,578
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost		1
Interest cost		$ 6

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

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

[3] (a)The pension actuarial gain in 2018 primarily reflects an increase in the discount rate. The OPEB actuarial gain in 2018 primarily reflects an increase in the discount rate and favorable health care claims experience. The pension and OPEB actuarial losses in 2017 primarily reflect a decrease in the discount rate.

[4] (b)Exelon recorded pension and OPEB obligations associated with its acquisition of Fitzpatrick on March 31, 2017.