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Stock-Based Compensation Plans - Schedule of Pre-Tax Stock-based Compensation Expense (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 23, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Allocated Share-based Compensation Expense |  |  | $ 208 | $ 191 | $ 175 | [1]
Exelon Generation Co L L C [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Allocated Share-based Compensation Expense |  |  | 77 | 88 | 78 |
Commonwealth Edison Co [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Allocated Share-based Compensation Expense |  |  | 8 | 7 | 8 |
PECO Energy Co [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Allocated Share-based Compensation Expense |  |  | 5 | 3 | 3 |
Baltimore Gas and Electric Company [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Allocated Share-based Compensation Expense |  |  | 3 | 1 | 1 |
Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Allocated Share-based Compensation Expense |  | $ 3 |  |  |  |
Corporate, Non-Segment [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Allocated Share-based Compensation Expense | [2] |  | 111 | 88 | 81 |
Corporate, Non-Segment [Member] | Pepco Holdings LLC [Member] | Successor [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Allocated Share-based Compensation Expense | [3],[4] |  | $ 4 | $ 4 | $ 4 |
[1] | (a)2016 amounts include expense related to stock-based compensation granted to eligible PHI employees since the merger date of March 23, 2016.
[2] | These amounts primarily represent amounts billed to Exelon’s subsidiaries through intercompany allocations. These amounts are not included in the Generation, ComEd, PECO, BGE or PHI amounts above.
[3] | Pepco's, DPL's and ACE's stock-based compensation expense for the years ended December 31, 2018 and 2017 was not material.
[4] | These amounts primarily represent amounts billed to PHI’s subsidiaries through PHISCO intercompany allocations.

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