Retirement Benefits Retirement Benefits - Components of Accumulated Other Comprehensive Income and Regulatory Assets (Liabilities) related to Retirement Plans (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2017	Mar. 23, 2016	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amounts recognized in regulatory assets, before tax		$ 57
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss				$ (222)	$ 644	[1]	$ 476
Amortization of actuarial loss				(607)	(554)	[1]	(571)
Current year prior service cost (credit)				9	(60)	[1]	0
Amortization of prior service (cost) credit				(1)	(14)	[1]	(13)
Settlements				3	0	[1]	2
DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeAndRegulatoryAssetsAcqusition				0	994	[1]	0
Total recognized in AOCI and regulatory assets (liabilities)	[1]			(824)	1,010		(110)
Defined benefit plan, amounts recognized in OCI, before tax				(401)	51	[1]	(64)
Defined benefit plan, amounts recognized in regulatory assets, before tax				(423)	959	[1]	(46)
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss			$ 0		50
Amortization of actuarial loss			(14)		(65)
Amortization of prior service (cost) credit			0		(2)
Total recognized in AOCI and regulatory assets (liabilities)	[1]		(14)		(17)
Defined benefit plan, amounts recognized in OCI, before tax			(1)		(11)
Defined benefit plan, amounts recognized in regulatory assets, before tax			(13)		(6)
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss				166	(101)	[1]	(194)
Amortization of actuarial loss				(61)	(63)	[1]	(80)
Current year prior service cost (credit)				0	0	[1]	(23)
Amortization of prior service (cost) credit				188	185	[1]	174
Settlements				0	0	[1]	0
DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeAndRegulatoryAssetsAcqusition				0	94	[1]	0
Total recognized in AOCI and regulatory assets (liabilities)	[1]			293	115		(123)
Defined benefit plan, amounts recognized in OCI, before tax				168	20	[1]	(63)
Defined benefit plan, amounts recognized in regulatory assets, before tax				$ 125	95	[1]	$ (60)
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss			0		(39)
Amortization of actuarial loss			(2)		(8)
Amortization of prior service (cost) credit			3		13
Total recognized in AOCI and regulatory assets (liabilities)	[1]		1		(34)
Defined benefit plan, amounts recognized in OCI, before tax			0		0
Defined benefit plan, amounts recognized in regulatory assets, before tax			$ 1		$ (34)

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