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Retirement Benefits Retirement Benefits - Components of Net Periodic Benefit Cost (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 Disclosure [Line Items] |  |  |  |  |  |  |
Net periodic benefit cost |  |  | $ 47 |  | $ 109 |  | $ 116
Pension Plan, Defined Benefit [Member] |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) | [1] |  | (1,561) |  | 1,182 |  |
Service cost |  |  | 405 | [2] | 387 | [3] | 354
Interest cost |  |  | 802 | [2] | 842 | [3] | 830
Expected return on assets |  |  | (1,252) | [2] | (1,196) | [3] | (1,141)
Prior service cost (credit) |  |  | 2 | [2] | 1 | [3] | 14
Actuarial loss |  |  | 629 | [2] | 607 | [3] | 554
Settlement and other charges(c) |  |  | 3 | [2],[4] | 3 | [3],[4] | 2
Net periodic benefit cost |  |  | 589 | [2] | 644 | [3] | 613
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Service cost |  | $ 12 |  |  |  |  |
Interest cost |  | 26 |  |  |  |  |
Expected return on assets |  | 30 |  |  |  |  |
Prior service cost (credit) |  | 0 |  |  |  |  |
Actuarial loss |  | (14) |  |  |  |  |
Net periodic benefit cost |  | 22 |  |  |  |  |
Other Postretirement Benefit Plan, Defined Benefit [Member] |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) | [1] |  | (540) |  | 350 |  |
Service cost |  |  | 112 | [2] | 106 | [3] | 107
Interest cost |  |  | 175 | [2] | 182 | [3] | 185
Expected return on assets |  |  | (173) | [2] | (162) | [3] | (162)
Prior service cost (credit) |  |  | (186) | [2] | (188) | [3] | (185)
Actuarial loss |  |  | 66 | [2] | 61 | [3] | 63
Settlement and other charges(c) |  |  | 1 | [2],[4] | 0 | [3],[4] | 0
Net periodic benefit cost |  |  | $ (5) | [2] | $ (1) | [3] | $ 8
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Service cost |  | 1 |  |  |  |  |
Interest cost |  | 6 |  |  |  |  |
Expected return on assets |  | 5 |  |  |  |  |
Prior service cost (credit) |  | (3) |  |  |  |  |
Actuarial loss |  | (2) |  |  |  |  |
Net periodic benefit cost |  | $ 1 |  |  |  |  |
[1] | (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.
[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] | 2016 amount includes an additional termination benefit for PHI.

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- Definition Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost recognized for passage of time related to defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(7) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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