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, 2016	Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amounts recognized in regulatory assets, before tax		$ 27
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate				4.04%	4.29%	3.94%
Current year actuarial loss (gain)				$ 644	$ 476	$ 1,639
Amortization of actuarial loss				(554)	(571)	(420)
Current year prior service (credit) cost				(60)	0	0
Amortization of prior service (cost) credit				(14)	(13)	(14)
DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeAndRegulatoryAssetsAcqusition				994	0	0
Settlements				0	(2)	(2)
Total recognized in AOCI and regulatory assets (liabilities)	[1]			1,010	(110)	1,203
Defined benefit plan, amounts recognized in OCI, before tax				51	(64)	788
Defined benefit plan, amounts recognized in regulatory assets, before tax				$ 959	$ (46)	$ 415
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate				4.04%	4.29%	3.92%
Current year actuarial loss (gain)				$ (101)	$ (194)	$ 561
Amortization of actuarial loss				(63)	(80)	(50)
Current year prior service (credit) cost				0	(23)	(1,012)
Amortization of prior service (cost) credit				185	174	122
DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeAndRegulatoryAssetsAcqusition				94	0	0
Settlements				0	0	0
Total recognized in AOCI and regulatory assets (liabilities)	[1]			115	(123)	(379)
Defined benefit plan, amounts recognized in OCI, before tax				20	(63)	(162)
Defined benefit plan, amounts recognized in regulatory assets, before tax				$ 95	(60)	$ (217)
Predecessor [Member] | Pepco Holdings LLC [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate						4.20%
Current year actuarial loss (gain)			$ 0		50	$ 276
Amortization of actuarial loss			(14)		(65)	(45)
Amortization of prior service (cost) credit			0		(2)	(2)
Total recognized in AOCI and regulatory assets (liabilities)	[1]		(14)		(17)	229
Defined benefit plan, amounts recognized in OCI, before tax			(1)		(11)	17
Defined benefit plan, amounts recognized in regulatory assets, before tax			(13)		$ (6)	$ 212
Predecessor [Member] | Pepco Holdings LLC [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate					4.55%	4.15%
Current year actuarial loss (gain)			0		$ (39)	$ 62
Amortization of actuarial loss			(2)		(8)	(3)
Amortization of prior service (cost) credit			3		13	13
Total recognized in AOCI and regulatory assets (liabilities)	[1]		1		(34)	72
Defined benefit plan, amounts recognized in OCI, before tax			0		0	0
Defined benefit plan, amounts recognized in regulatory assets, before tax			$ 1		$ (34)	$ 72

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