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Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities - Fair Value Inputs Assets Quantitative Information (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Fair Value, Inputs, Level 3 [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Cash Collateral Posted |  | $ 76,000,000 |  | $ 81,000,000 |
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets, fair value | [1],[2] | 443,000,000 |  | 445,000,000 |
Derivative liabilities, fair value |  | $ (249,000,000) |  | $ (256,000,000) |
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Forward power price assets |  | 12 |  | 3 |
Forward gas price assets |  | 0.78 |  | 1.27 |
Marketability Reserve |  | 4.00% |  | 4.00% |
Renewable factor |  | 86.00% |  | 88.00% |
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Forward power price assets |  | 174 |  | 124 |
Forward gas price assets |  | 12.38 |  | 12.8 |
Marketability Reserve |  | 8.00% |  | 8.00% |
Renewable factor |  | 120.00% |  | 120.00% |
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Minimum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Volatility percentage |  | 10.00% |  | 11.00% |
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Maximum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Volatility percentage |  | 277.00% |  | 139.00% |
Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative liabilities, fair value | [1],[2] | $ 56,000,000 |  | $ 26,000,000 |
Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Minimum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Forward power price assets |  | 14 |  | 14 |
Exelon Generation Co L L C [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Cash Collateral Posted |  | $ 418,000,000 |  | $ 497,000,000 |
Derivative liabilities, fair value | [3] | 618,000,000 |  | 349,000,000 |
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative assets, fair value |  | $ 443,000,000 | [1],[2] | $ 445,000,000 | [4],[5]
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Forward power price assets |  | 12 |  | 3 |
Forward gas price assets |  | 0.78 |  | 1.27 |
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Forward power price assets |  | 174 |  | 124 |
Forward gas price assets |  | 12.38 |  | 12.8 |
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Minimum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Volatility percentage |  | 10.00% |  | 11.00% |
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Maximum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Volatility percentage |  | 277.00% |  | 139.00% |
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative liabilities, fair value |  | $ 56,000,000 | [1],[2] | $ 26,000,000 | [4],[5]
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Minimum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Forward power price assets |  | 14 |  | 14 |
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Maximum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Forward power price assets |  | 174 |  | 94 |
Commonwealth Edison Co [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Derivative liabilities, fair value |  | $ (249,000,000) |  | $ (256,000,000) |
Commonwealth Edison Co [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Marketability Reserve |  | 4.00% |  | 4.00% |
Renewable factor |  | 86.00% |  | 88.00% |
Commonwealth Edison Co [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member] |  |  |  |  |
Fair Value Inputs [Abstract] |  |  |  |  |
Marketability Reserve |  | 8.00% |  | 8.00% |
Renewable factor |  | 120.00% |  | 120.00% |
[1] | The fair values do not include cash collateral posted on level three positions of $76 million as of December 31, 2018.
[2] | The valuation techniques, unobservable inputs and ranges are the same for the asset and liability positions.
[3] | Amount represents the net fair value of out-of-the-money derivative contracts containing credit-risk related contingent features after considering the mitigating effects of offsetting positions under master netting arrangements and reflects the actual net liability upon which any potential contingent collateral obligations would be based.
[4] | The fair values do not include cash collateral posted on level three positions $81 million as of December 31, 2017.
[5] | The valuation techniques, unobservable inputs and ranges are the same for the asset and liability positions

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