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Retirement Benefits Retirement Benefits - Allocated Portion of Pension and Postretirement Benefit Plan Costs (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 23, 2016 | Dec. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense |  |  |  | $ 583 |  | $ 643 |  | $ 621 |
Corporate, Non-Segment [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense | [1] |  |  | 57 |  | 53 |  | 48 |
Exelon Generation Co L L C [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense | [2] |  |  | 204 |  | 227 |  | 218 |
Commonwealth Edison Co [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense |  |  |  | 177 |  | 176 |  | 166 |
PECO Energy Co [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense |  |  |  | 18 |  | 29 |  | 33 |
Baltimore Gas and Electric Company [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense |  |  |  | 60 |  | 64 |  | 68 |
Pepco Holdings LLC [Member] | Successor [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense |  |  | $ 67 |  |  |  |  |  |
Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense |  | $ 94 |  |  |  | 88 |  | 23 |
Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense |  | 9 |  | 34 | [3],[4] | 43 | [3],[4] | 47 | [3],[4]
Potomac Electric Power Company [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense |  | 7 |  | 15 | [3] | 25 | [3] | 31 | [3]
Delmarva Power & Light Company [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense |  | 4 |  | 6 | [3] | 13 | [3] | 18 | [3]
Atlantic City Electric Company [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Amount included in capital and operating & maintenance expense |  | $ 3 |  | $ 12 | [3] | $ 13 | [3] | $ 15 | [3]
[1] | These amounts primarily represent amounts billed to Exelon’s subsidiaries through intercompany allocations. These amounts are not included in the Generation, ComEd, PECO, BGE, PHI, Pepco, DPL or ACE amounts above.
[2] | FitzPatrick net benefit costs are included for the period after acquisition
[3] | Pepco's, DPL's, ACE's and PHISCO's pension and postretirement benefit costs for the year ended December 31, 2016 include $7 million, $4 million, $3 million and $9 million, respectively, of costs incurred prior to the closing of Exelon’s merger with PHI on March 23, 2016.
[4] | These amounts represent amounts billed to Pepco, DPL and ACE through intercompany allocations. These amounts are not included in Pepco, DPL or ACE amounts above

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- Definition The amount of pension and other (such as medical, dental and life insurance) postretirement benefit costs included in capital and operating and maintenance expense that was recognized during the period for defined benefit plans (periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss on assets, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments). + References No definition available. + Details
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