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Property, Plant and Equipment Property, Plant and Equipment - Summary of Capitalized Software 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
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Net |  |  | $ 810 | $ 834 |
Capitalized Computer Software, Amortization |  |  | 282 | 270 | $ 255
Exelon Generation Co L L C [Member] |  |  |  |  |
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Net |  |  | 164 | 173 |
Capitalized Computer Software, Amortization |  |  | 78 | 73 | 72
Commonwealth Edison Co [Member] |  |  |  |  |
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Net |  |  | 206 | 227 |
Capitalized Computer Software, Amortization |  |  | 79 | 73 | 62
PECO Energy Co [Member] |  |  |  |  |
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Net |  |  | 98 | 111 |
Capitalized Computer Software, Amortization |  |  | 37 | 39 | 33
Baltimore Gas and Electric Company [Member] |  |  |  |  |
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Net |  |  | 166 | 179 |
Capitalized Computer Software, Amortization |  |  | 48 | 46 | 44
Potomac Electric Power Company [Member] |  |  |  |  |
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Net |  |  | 26 | 2 |
Capitalized Computer Software, Amortization |  |  | 2 | 0 | 0
Pepco Holdings LLC [Member] |  |  |  |  |
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Net |  |  | 165 | 133 |
Capitalized Computer Software, Amortization |  |  | 33 | 34 |
Delmarva Power & Light Company [Member] |  |  |  |  |
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Net |  |  | 21 | 1 |
Capitalized Computer Software, Amortization |  |  | 2 | 0 | 0
Atlantic City Electric Company [Member] |  |  |  |  |
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Net |  |  | 14 | 1 |
Capitalized Computer Software, Amortization |  |  | $ 1 | $ 0 | $ 0
Successor [Member] | Pepco Holdings LLC [Member] |  |  |  |  |
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Amortization |  | $ 29 |  |  |
Predecessor [Member] | Pepco Holdings LLC [Member] |  |  |  |  |
Capitalized Software [Line Items] |  |  |  |  |
Capitalized Computer Software, Amortization | $ 8 |  |  |  |

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