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Retirement Benefits Retirement Benefits - Contributions made to Pension and Other Postretirement Benefit Plans (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 Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | $ 383 |  | $ 405 |  | $ 397 |
Expected qualified pension plan contributions |  |  |  |  | 301 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 25 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 44 |  |  |  |  |
Corporate, Non-Segment [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Expected qualified pension plan contributions |  |  |  |  | 41 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 7 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 2 |  |  |  |  |
Constellation Legacy Benefit Plans [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [1] |  |  |  | 0 |  | 21 |  | 25 |
Exelon Generation Co L L C [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 139 |  | 148 |  | 152 |
Expected qualified pension plan contributions |  |  |  |  | 135 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 7 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 13 |  |  |  |  |
Commonwealth Edison Co [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 42 |  | 41 |  | 38 |
Expected qualified pension plan contributions |  |  |  |  | 65 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 1 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 2 |  |  |  |  |
PECO Energy Co [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 28 |  | 24 |  | 30 |
Expected qualified pension plan contributions |  |  |  |  | 25 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 1 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 0 |  |  |  |  |
Baltimore Gas and Electric Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 54 |  | 53 |  | 49 |
Expected qualified pension plan contributions |  |  |  |  | 34 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 1 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 15 |  |  |  |  |
Pepco Holdings LLC [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | (74) |  | (99) |  |  |
Expected qualified pension plan contributions |  |  |  |  | 1 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 8 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 12 |  |  |  |  |
Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Expected qualified pension plan contributions |  |  |  |  | 1 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 5 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 1 |  |  |  |  |
Pepco Holdings LLC [Member] | Successor [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  | $ (86) |  |  |  |  |  |
Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  | $ (4) |  |  |  |  |  |  |  |
Potomac Electric Power Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 17 |  | 72 |  | 32 |
Expected qualified pension plan contributions |  |  |  |  | 0 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 2 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 10 |  |  |  |  |
Delmarva Power & Light Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 0 |  | 2 |  | 22 |
Expected qualified pension plan contributions |  |  |  |  | 0 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 1 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 0 |  |  |  |  |
Atlantic City Electric Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 6 |  | 20 |  | 17 |
Expected qualified pension plan contributions |  |  |  |  | 0 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 0 |  |  |  |  |
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  |  |  | 1 |  |  |  |  |
Pension Plan, Defined Benefit [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 337 | [2] | 341 | [2] | 347 |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 1,196 |  |  |  |  |
Pension Plan, Defined Benefit [Member] | Corporate, Non-Segment [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [1],[2] |  |  |  | 41 |  | 38 |  | 39 |
Pension Plan, Defined Benefit [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [2] |  |  |  | 128 |  | 137 |  | 140 |
Pension Plan, Defined Benefit [Member] | Commonwealth Edison Co [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [2] |  |  |  | 38 |  | 36 |  | 33 |
Pension Plan, Defined Benefit [Member] | PECO Energy Co [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [2] |  |  |  | 28 |  | 24 |  | 30 |
Pension Plan, Defined Benefit [Member] | Baltimore Gas and Electric Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [2] |  |  |  | 40 |  | 39 |  | 31 |
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  | 67 | 62 |  |  |  |  |
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  | 4 | [1] |  | 50 | [2],[3] | 5 | [2],[3] | 17 | [2],[3]
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Successor [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  | 74 |  |  |  |  |  |  |  |
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  |  |  | 4 |  |  |
Pension Plan, Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 2 | [1] | 4 |  | 6 |
Pension Plan, Defined Benefit [Member] | Potomac Electric Power Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [2] |  |  |  | 6 |  | 62 |  | 24 |
Pension Plan, Defined Benefit [Member] | Delmarva Power & Light Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [2] |  |  |  | 0 |  | 0 |  | 22 |
Pension Plan, Defined Benefit [Member] | Atlantic City Electric Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [2] |  |  |  | 6 |  | 0 |  | 15 |
Other Postretirement Benefit Plan, Defined Benefit [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 46 |  | 64 |  | 50 |
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months |  |  |  |  | 255 |  |  |  |  |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Corporate, Non-Segment [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [1] |  |  |  | 5 |  | 2 |  | 3 |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 11 |  | 11 |  | 12 |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Commonwealth Edison Co [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 4 |  | 5 |  | 5 |
Other Postretirement Benefit Plan, Defined Benefit [Member] | PECO Energy Co [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 0 |  | 0 |  | 0 |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Baltimore Gas and Electric Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 14 |  | 14 |  | 18 |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  | $ 32 | 12 |  |  |  |  |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions | [3] |  |  |  | 1 |  | 0 |  | 2 |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Successor [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  | $ 12 |  |  |  |  |  |  |  |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  |  |  | 0 |  |  |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Potomac Electric Power Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 11 |  | 10 |  | 8 |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Delmarva Power & Light Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | 0 |  | 2 |  | 0 |
Other Postretirement Benefit Plan, Defined Benefit [Member] | Atlantic City Electric Company [Member] |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |
Pension and non-pension postretirement benefit contributions |  |  |  |  | $ 0 |  | $ 20 |  | $ 2 |
[1] | )Includes $2 million, $4 million, and $6 million of pension contributions funded by Exelon Corporate, for the years ended December 31, 2018, 2017, and 2016, respectively.
[2] | Exelon's and Generation's pension contributions include $21 million and $25 million related to the legacy CENG plans that was funded by CENG as provided in an Employee Matters Agreement (EMA) between Exelon and CENG for the years ended December 31, 2017 and 2016, respectively
[3] | PHISCO’s pension contributions for the year ended December 31, 2016 include $4 million of contributions made prior to the closing of Exelon’s merger with PHI on March 23, 2016

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- Definition The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the other postretirement benefit plan during the next fiscal year beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References No definition available. + Details
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- Definition The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the qualified pension plan during the next fiscal year beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References No definition available. + Details
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