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, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Employer contributions						$ 13		$ 9
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				$ 34		0	[1]
Net benefit obligation at beginning of year	[1]	$ 17,753		21,060		17,753
Service cost				387	[2]	354	[1],[3]	326
Interest cost				842	[2]	830	[1],[3]	710
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant				0		0	[1]
Actuarial loss (gain)				1,182		567	[1]
Plan amendments				9		(60)	[1]
Acquisitions/divestitures(b)	[4]			16		2,667	[1]
Defined Benefit Plan, Benefit Obligation, Benefits Paid				(1,125)		(1,051)	[1]
Net benefit obligation at end of year				22,337		21,060	[1]	17,753	[1]
Fair value of net plan assets at beginning of year	[1]	14,347		16,791		14,347
Actual return on plan assets				2,600		1,061	[1]
Employer contributions				341		347	[1]
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant				0		0	[1]
Defined Benefit Plan, Plan Assets, Benefits Paid				(1,125)		(1,051)	[1]
Acquisitions/divestitures(b)	[4]			0		2,087	[1]
Defined Benefit Plan, Fair Value of Plan Assets				18,573		16,791	[1]	14,347	[1]
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year				2,490
Service cost		12				57
Interest cost		26				109
Actuarial loss (gain)		(30)
Defined Benefit Plan, Benefit Obligation, Benefits Paid		(2)
Net benefit obligation at end of year		2,496				2,490
Fair value of net plan assets at beginning of year				2,018
Employer contributions		4
Defined Benefit Plan, Plan Assets, Benefits Paid		(2)
Defined Benefit Plan, Fair Value of Plan Assets		2,020				2,018
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				0		0	[1]
Net benefit obligation at beginning of year		3,938	[1]	4,457		3,938	[1]
Service cost				106	[2]	107	[1],[3]	119
Interest cost				182	[2]	185	[1],[3]	167
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant				53		54	[1]
Actuarial loss (gain)				350		(136)	[1]
Plan amendments				0		0	[1]
Acquisitions/divestitures(b)	[4]			17		589	[1]
Defined Benefit Plan, Benefit Obligation, Benefits Paid				(309)		(280)	[1]
Net benefit obligation at end of year				4,856		4,457		3,938	[1]
Fair value of net plan assets at beginning of year	[1]	2,293		2,578		2,293
Actual return on plan assets				346		128	[1]
Employer contributions				64		50	[1]
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant				53		54	[1]
Defined Benefit Plan, Plan Assets, Benefits Paid				(309)		(280)	[1]
Acquisitions/divestitures(b)	[4]			0		333	[1]
Defined Benefit Plan, Fair Value of Plan Assets				2,732		2,578	[1]	$ 2,293	[1]
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year				563
Service cost		1				7
Interest cost		6				24
Actuarial loss (gain)		(5)
Defined Benefit Plan, Benefit Obligation, Benefits Paid		(1)
Net benefit obligation at end of year		564				563
Fair value of net plan assets at beginning of year				$ 348
Employer contributions		1
Defined Benefit Plan, Plan Assets, Benefits Paid		(1)
Defined Benefit Plan, Fair Value of Plan Assets		$ 348				$ 348

[1] 2016 amounts include PHI for the period of March 24, 2016 through December 31, 2016.

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

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

[4] Exelon recorded pension and OPEB obligations associated with its acquisition of Fitzpatrick on March 31, 2017. Effective March 23, 2016, Exelon became the sponsor of PHI's defined benefit pension and other postretirement benefit plans.