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Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income Taxes [Line Items] |  |  |
Unrecognized Tax Benefits, Increase Resulting from Acquisition |  |  | $ 22
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits - beginning balance | $ 743 | $ 916 | 1,078
Increases based on tax positions related to current year |  |  | 108
Increases based on tax positions prior to current year | 15 |  | (332)
Decreases based on tax positions prior to current year | 30 | 28 | 88
Unrecognized Tax Benefits, Period Increase (Decrease) | (251) | (196) | (21)
Decrease from settlements with taxing authorities | (53) | (5) | (27)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations | (7) |  |
Unrecognized tax benefits - ending balance | 477 | 743 | 916
Exelon Generation Co L L C [Member] |  |  |
Income Taxes [Line Items] |  |  |
Unrecognized Tax Benefits, Increase Resulting from Acquisition |  |  | 5
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits - beginning balance | 468 | 490 | 534
Increases based on tax positions related to current year |  |  | 10
Increases based on tax positions prior to current year | 15 |  | (12)
Decreases based on tax positions prior to current year | 21 | 0 | 0
Unrecognized Tax Benefits, Period Increase (Decrease) | (36) | (17) | (20)
Decrease from settlements with taxing authorities | (53) | (5) | (27)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations | (7) |  |
Unrecognized tax benefits - ending balance | 408 | 468 | 490
Commonwealth Edison Co [Member] |  |  |
Income Taxes [Line Items] |  |  |
Unrecognized Tax Benefits, Increase Resulting from Acquisition |  |  | 0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits - beginning balance | 2 | (12) | 142
Increases based on tax positions related to current year |  |  | 0
Increases based on tax positions prior to current year | 0 |  | (154)
Decreases based on tax positions prior to current year | 0 | 14 | 0
Unrecognized Tax Benefits, Period Increase (Decrease) | 0 | 0 | 0
Decrease from settlements with taxing authorities | 0 | 0 | 0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations | 0 |  |
Unrecognized tax benefits - ending balance | 2 | 2 | (12)
PECO Energy Co [Member] |  |  |
Income Taxes [Line Items] |  |  |
Unrecognized Tax Benefits, Increase Resulting from Acquisition |  |  | 0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits - beginning balance | 0 | 0 | 0
Increases based on tax positions related to current year |  |  | 0
Increases based on tax positions prior to current year | 0 |  | 0
Decreases based on tax positions prior to current year | 0 | 0 | 0
Unrecognized Tax Benefits, Period Increase (Decrease) | 0 | 0 | 0
Decrease from settlements with taxing authorities | 0 | 0 | 0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations | 0 |  |
Unrecognized tax benefits - ending balance | 0 | 0 | 0
Baltimore Gas and Electric Company [Member] |  |  |
Income Taxes [Line Items] |  |  |
Unrecognized Tax Benefits, Increase Resulting from Acquisition |  |  | 0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits - beginning balance | 120 | 120 | 120
Increases based on tax positions related to current year |  |  | 0
Increases based on tax positions prior to current year | 0 |  | 0
Decreases based on tax positions prior to current year | 0 | 0 | 0
Unrecognized Tax Benefits, Period Increase (Decrease) | (120) | 0 | 0
Decrease from settlements with taxing authorities | 0 | 0 | 0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations | 0 |  |
Unrecognized tax benefits - ending balance | 0 | 120 | 120
Potomac Electric Power Company [Member] |  |  |
Income Taxes [Line Items] |  |  |
Unrecognized Tax Benefits, Increase Resulting from Acquisition |  |  | 0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits - beginning balance | 59 | 80 | 8
Increases based on tax positions related to current year |  |  | 21
Increases based on tax positions prior to current year | 0 |  | 0
Decreases based on tax positions prior to current year | 7 | 0 | 51
Unrecognized Tax Benefits, Period Increase (Decrease) | (66) | (21) | 0
Decrease from settlements with taxing authorities | 0 | 0 | 0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations | 0 |  |
Unrecognized tax benefits - ending balance | 0 | 59 | 80
Delmarva Power & Light Company [Member] |  |  |
Income Taxes [Line Items] |  |  |
Unrecognized Tax Benefits, Increase Resulting from Acquisition |  |  | 0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits - beginning balance | 21 | 37 | 3
Increases based on tax positions related to current year |  |  | 16
Increases based on tax positions prior to current year | 0 |  | 0
Decreases based on tax positions prior to current year | 1 | 0 | 18
Unrecognized Tax Benefits, Period Increase (Decrease) | (22) | (16) | 0
Decrease from settlements with taxing authorities | 0 | 0 | 0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations | 0 |  |
Unrecognized tax benefits - ending balance | 0 | 21 | 37
Atlantic City Electric Company [Member] |  |  |
Income Taxes [Line Items] |  |  |
Unrecognized Tax Benefits, Increase Resulting from Acquisition |  |  | 0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits - beginning balance | 14 | 22 | 0
Increases based on tax positions related to current year |  |  | 22
Increases based on tax positions prior to current year | 0 |  | 0
Decreases based on tax positions prior to current year | 0 | 14 | 0
Unrecognized Tax Benefits, Period Increase (Decrease) | 0 | (22) | 0
Decrease from settlements with taxing authorities | 0 | 0 | 0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations | 0 |  |
Unrecognized tax benefits - ending balance | 14 | 14 | 22
Pepco Holdings LLC [Member] |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits - beginning balance | 125 | 172 |
Increases based on tax positions prior to current year | 0 |  |
Decreases based on tax positions prior to current year | 8 | 14 |
Unrecognized Tax Benefits, Period Increase (Decrease) | (88) | (61) |
Decrease from settlements with taxing authorities | 0 | 0 |
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations | 0 |  |
Unrecognized tax benefits - ending balance | $ 45 | 125 | 172
Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |
Income Taxes [Line Items] |  |  |
Unrecognized Tax Benefits, Increase Resulting from Acquisition |  |  | (5)
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Unrecognized tax benefits - beginning balance |  | $ 172 | 22
Increases based on tax positions related to current year |  |  | 59
Increases based on tax positions prior to current year |  |  | 0
Decreases based on tax positions prior to current year |  |  | 96
Unrecognized Tax Benefits, Period Increase (Decrease) |  |  | 0
Decrease from settlements with taxing authorities |  |  | 0
Unrecognized tax benefits - ending balance |  |  | $ 172

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