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Property, Plant and Equipment Property, Plant and Equipment - Summary of Capitalized Interest and AFUDC (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 Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] |  |  | $ 1,695 | $ 1,658 | $ 1,678
Interest Costs Capitalized |  |  |  | 31 | 63 | 108
Allowance for Funds Used During Construction, Capitalized Interest |  |  |  | 109 | 108 | 98
Pepco Holdings LLC [Member] |  |  |  |  |  |
Capitalized Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] |  |  | 305 | 263 |
Allowance for Funds Used During Construction, Capitalized Interest |  |  |  | 44 | 54 |
Exelon Generation Co L L C [Member] |  |  |  |  |  |
Capitalized Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] |  |  | 464 | 502 | 472
Interest Costs Capitalized |  |  |  | 31 | 63 | 107
Allowance for Funds Used During Construction, Capitalized Interest |  |  |  | 0 | 0 | 0
Commonwealth Edison Co [Member] |  |  |  |  |  |
Capitalized Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] |  |  | 377 | 369 | 469
Interest Costs Capitalized |  |  |  | 0 | 0 | 0
Allowance for Funds Used During Construction, Capitalized Interest |  |  |  | 30 | 20 | 22
PECO Energy Co [Member] |  |  |  |  |  |
Capitalized Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] |  |  | 141 | 130 | 127
Interest Costs Capitalized |  |  |  | 0 | 0 | 0
Allowance for Funds Used During Construction, Capitalized Interest |  |  |  | 12 | 12 | 11
Baltimore Gas and Electric Company [Member] |  |  |  |  |  |
Capitalized Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] |  |  | 130 | 111 | 114
Interest Costs Capitalized |  |  |  | 0 | 0 | 0
Allowance for Funds Used During Construction, Capitalized Interest |  |  |  | 24 | 22 | 30
Potomac Electric Power Company [Member] |  |  |  |  |  |
Capitalized Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] |  |  | 162 | 133 | 137
Interest Costs Capitalized |  |  |  | 0 | 0 | 0
Allowance for Funds Used During Construction, Capitalized Interest |  |  |  | 34 | 34 | 29
Delmarva Power & Light Company [Member] |  |  |  |  |  |
Capitalized Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] |  |  | 62 | 54 | 52
Interest Costs Capitalized |  |  |  | 0 | 0 | 0
Allowance for Funds Used During Construction, Capitalized Interest |  |  |  | 4 | 10 | 7
Atlantic City Electric Company [Member] |  |  |  |  |  |
Capitalized Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] |  |  | 68 | 64 | 65
Interest Costs Capitalized |  |  |  | 0 | 0 | 0
Allowance for Funds Used During Construction, Capitalized Interest |  |  |  | $ 4 | $ 9 | $ 9
Successor [Member] | Pepco Holdings LLC [Member] |  |  |  |  |  |
Capitalized Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] |  | $ 207 |  |  |
Allowance for Funds Used During Construction, Capitalized Interest |  |  | $ 35 |  |  |
Predecessor [Member] | Pepco Holdings LLC [Member] |  |  |  |  |  |
Capitalized Interest And AFUDC [Line Items] |  |  |  |  |  |
Interest Costs Incurred | [1] | $ 68 |  |  |  |
Allowance for Funds Used During Construction, Capitalized Interest |  | $ 10 |  |  |  |
[1] | Includes interest expense to affiliates.

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