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Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
Significant Accounting Policies Additional Narrative Information [Line Items] |  |
Stockholders' Equity Attributable to Noncontrolling Interest | $ 2,306 | $ 2,291
Percentage ownership of consolidated subsidiaries | 100.00% |
Minimum expectation of tax position to be realized | 50.00% |
Commonwealth Edison Company [Member] |  |
Significant Accounting Policies Additional Narrative Information [Line Items] |  |
Stockholders' Equity Attributable to Noncontrolling Interest |  | $ 1
Percentage ownership of consolidated subsidiaries | 99.00% |
CENG [Member] |  |
Significant Accounting Policies Additional Narrative Information [Line Items] |  |
Ownership interest | 50.01% |
ExGen Renewables [Member] |  |
Significant Accounting Policies Additional Narrative Information [Line Items] |  |
Ownership interest | 51.00% |
Minimum [Member] |  |
Significant Accounting Policies Additional Narrative Information [Line Items] |  |
Ownership interest | 20.00% |
Maximum [Member] |  |
Significant Accounting Policies Additional Narrative Information [Line Items] |  |
Ownership interest | 50.00% |

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