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Income Taxes - Summary of Interest Expense (Income) (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
Income Taxes [Line Items] |  |  |  |  |
Unrecognized Tax Benefits, Income Tax Penalties Expense |  |  | $ 0 | $ (2) | $ 106
Net interest (income) expense recognized related to uncertain tax positions |  |  | (3) | 37 | 165
Exelon Generation Co L L C [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Unrecognized Tax Benefits, Income Tax Penalties Expense |  |  | 0 | 0 | 0
Net interest (income) expense recognized related to uncertain tax positions |  |  | 0 | (1) | (13)
Commonwealth Edison Co [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Unrecognized Tax Benefits, Income Tax Penalties Expense |  |  | 0 | 0 | 86
Net interest (income) expense recognized related to uncertain tax positions |  |  | 0 | 11 | 117
PECO Energy Co [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Unrecognized Tax Benefits, Income Tax Penalties Expense |  |  | 0 | 0 | 0
Net interest (income) expense recognized related to uncertain tax positions |  |  | 0 | 0 | 0
Baltimore Gas and Electric Company [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Unrecognized Tax Benefits, Income Tax Penalties Expense |  |  | 0 | 0 | 0
Net interest (income) expense recognized related to uncertain tax positions |  |  | 0 | 0 | 0
Pepco Holdings LLC [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Net interest (income) expense recognized related to uncertain tax positions |  |  | 0 |  |
Pepco Holdings LLC [Member] | Successor [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Net interest (income) expense recognized related to uncertain tax positions |  | $ (2) |  |  |
Pepco Holdings LLC [Member] | Predecessor [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Net interest (income) expense recognized related to uncertain tax positions | $ 0 |  |  |  |
Potomac Electric Power Company [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Unrecognized Tax Benefits, Income Tax Penalties Expense |  |  | 0 | 0 | 0
Net interest (income) expense recognized related to uncertain tax positions |  |  | 0 | 0 | 6
Delmarva Power & Light Company [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Unrecognized Tax Benefits, Income Tax Penalties Expense |  |  | 0 | 0 | 0
Net interest (income) expense recognized related to uncertain tax positions |  |  | 0 | 0 | 0
Atlantic City Electric Company [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Unrecognized Tax Benefits, Income Tax Penalties Expense |  |  | 0 | 0 | 0
Net interest (income) expense recognized related to uncertain tax positions |  |  | $ 0 | $ 0 | $ (1)

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