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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, 2018 | Mar. 23, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Defined benefit plan, amounts recognized in regulatory assets, before tax |  | $ 61 |  |  |  |  |
Pension Plan, Defined Benefit [Member] |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Current year actuarial (gain) loss |  |  |  | $ 635 | $ (222) | [1] | $ 644
Amortization of actuarial loss |  |  |  | (629) | (607) | [1] | (554)
Current year prior service cost (credit) |  |  |  | (4) | 9 | [1] | (60)
Amortization of prior service (cost) credit |  |  |  | (2) | (1) | [1] | (14)
Settlements |  |  |  | 3 | (3) | [1] | 0
DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeAndRegulatoryAssetsAcqusition |  |  |  | 0 | 0 | [1] | 994
Total recognized in AOCI and regulatory assets (liabilities) | [1] |  |  | (3) | (824) |  | 1,010
Defined benefit plan, amounts recognized in OCI, before tax |  |  |  | 3 | (401) | [1] | 51
Defined benefit plan, amounts recognized in regulatory assets, before tax |  |  |  | (6) | (423) | [1] | 959
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Current year actuarial (gain) loss |  |  | $ 0 |  |  |  |
Amortization of actuarial loss |  |  | (14) |  |  |  |
Amortization of prior service (cost) credit |  |  | 0 |  |  |  |
Total recognized in AOCI and regulatory assets (liabilities) | [1] |  | (14) |  |  |  |
Defined benefit plan, amounts recognized in OCI, before tax |  |  | (1) |  |  |  |
Defined benefit plan, amounts recognized in regulatory assets, before tax |  |  | (13) |  |  |  |
Other Postretirement Benefit Plan, Defined Benefit [Member] |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Current year actuarial (gain) loss |  |  |  | (232) | 166 | [1] | (101)
Amortization of actuarial loss |  |  |  | (66) | (61) | [1] | (63)
Current year prior service cost (credit) |  |  |  | 0 | 0 | [1] | 0
Amortization of prior service (cost) credit |  |  |  | 186 | 188 | [1] | 185
Settlements |  |  |  | 0 | 0 | [1] | 0
DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeAndRegulatoryAssetsAcqusition |  |  |  | 0 | 0 | [1] | 94
Total recognized in AOCI and regulatory assets (liabilities) | [1] |  |  | (112) | 293 |  | 115
Defined benefit plan, amounts recognized in OCI, before tax |  |  |  | (55) | 168 | [1] | 20
Defined benefit plan, amounts recognized in regulatory assets, before tax |  |  |  | $ (57) | $ 125 | [1] | $ 95
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 |  |  |  |
Amortization of actuarial loss |  |  | (2) |  |  |  |
Amortization of prior service (cost) credit |  |  | 3 |  |  |  |
Total recognized in AOCI and regulatory assets (liabilities) | [1] |  | 1 |  |  |  |
Defined benefit plan, amounts recognized in OCI, before tax |  |  | 0 |  |  |  |
Defined benefit plan, amounts recognized in regulatory assets, before tax |  |  | $ 1 |  |  |  |
[1] | 2016 amounts include PHI for the period of March 24, 2016 through December 31, 2016.

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- Definition The pretax total of net (gain) loss, prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost. + References No definition available. + Details
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