| **Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands** | Mar. 31, 2022 | Dec. 31, 2021 |
|-----------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Assets**                                                                                                            |          |          |
| Commodity contracts, assets                                                                                           | $ 297,623     | $ 110,389     |
| Commodity contracts, liabilities                                                                                      | (8,621)       | (4,690)       |
| Nuclear decommissioning trust                                                                                         | 1,227,465     | 1,294,757     |
| Nuclear decommissioning trust, other                                                                                  | 568,428       | 567,266       |
| Other special use funds                                                                                               | 349,042       | 358,410       |
| Other special use funds, other                                                                                        | 1,112         | 936           |
| Total assets                                                                                                          | 1,874,130     | 1,763,556     |
| Total assets, other                                                                                                   | 560,919       | 563,512       |
| **Liabilities**                                                                                                       |          |          |
| Gross derivative liability                                                                                            | (1,554)       |          |
| Gross derivative liability, other                                                                                     |          | 3,105         |
| Amount reported on balance sheet                                                                                 | (1,706)       | (4,373)       |
| Equity securities                                                                                                     |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 17,265        | 17,482        |
| Nuclear decommissioning trust, other                                                                                  | 478           | (27,782)      |
| Other special use funds                                                                                               | 28,180        | 48,506        |
| Other special use funds, other                                                                                        | 1,112         | 936           |
| U.S. commingled equity funds                                                                                          |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 567,950       | 595,048       |
| U.S. Treasury debt                                                                                                    |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 225,902       | 240,745       |
| Other special use funds                                                                                               | 312,613       | 298,170       |
| Corporate debt                                                                                                        |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 196,300       | 203,454       |
| Mortgage-backed securities                                                                                            |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 145,845       | 155,574       |
| Municipal bonds                                                                                                       |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 65,494        | 72,189        |
| Other special use funds                                                                                               | 8,249         | 11,734        |
| Other fixed income                                                                                                    |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 8,709         | 10,265        |
| Level 1                                                                                                          |          |          |
| **Assets**                                                                                                            |          |          |
| Commodity contracts, assets                                                                                           | 0             | 0             |
| Nuclear decommissioning trust                                                                                         | 242,689       | 286,009       |
| Other special use funds                                                                                               | 339,681       | 345,740       |
| Total assets                                                                                                          | 582,370       | 631,749       |
| **Liabilities**                                                                                                       |          |          |
| Gross derivative liability                                                                                            | 0             | 0             |
| Level 1 | Equity securities                                                                                      |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 16,787        | 45,264        |
| Other special use funds                                                                                               | 27,068        | 47,570        |
| Level 1 | U.S. commingled equity funds                                                                           |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Level 1 | U.S. Treasury debt                                                                                     |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 225,902       | 240,745       |
| Other special use funds                                                                                               | 312,613       | 298,170       |
| Level 1 | Corporate debt                                                                                         |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Level 1 | Mortgage-backed securities                                                                             |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Level 1 | Municipal bonds                                                                                        |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Other special use funds                                                                                               |               | 0             |
| Level 1 | Other fixed income                                                                                     |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Level 2                                                                                                          |          |          |
| **Assets**                                                                                                            |          |          |
| Commodity contracts, assets                                                                                           | 296,442       | 115,079       |
| Nuclear decommissioning trust                                                                                         | 416,348       | 441,482       |
| Other special use funds                                                                                               | 8,249         | 11,734        |
| Total assets                                                                                                          | 721,039       | 568,295       |
| **Liabilities**                                                                                                       |          |          |
| Gross derivative liability                                                                                            | 0             | (4,740)       |
| Level 2 | Equity securities                                                                                      |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Other special use funds                                                                                               | 0             | 0             |
| Level 2 | U.S. commingled equity funds                                                                           |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Level 2 | U.S. Treasury debt                                                                                     |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Other special use funds                                                                                               | 0             | 0             |
| Level 2 | Corporate debt                                                                                         |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 196,300       | 203,454       |
| Level 2 | Mortgage-backed securities                                                                             |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 145,845       | 155,574       |
| Level 2 | Municipal bonds                                                                                        |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 65,494        | 72,189        |
| Other special use funds                                                                                               | 8,249         | 11,734        |
| Level 2 | Other fixed income                                                                                     |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 8,709         | 10,265        |
| Level 3                                                                                                          |          |          |
| **Assets**                                                                                                            |          |          |
| Commodity contracts, assets                                                                                           | 9,802         | 0             |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Other special use funds                                                                                               | 0             | 0             |
| Total assets                                                                                                          | 9,802         | 0             |
| **Liabilities**                                                                                                       |          |          |
| Gross derivative liability                                                                                            | (152)         | (2,738)       |
| Level 3 | Equity securities                                                                                      |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Other special use funds                                                                                               | 0             | 0             |
| Level 3 | U.S. commingled equity funds                                                                           |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Level 3 | U.S. Treasury debt                                                                                     |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Other special use funds                                                                                               | 0             | 0             |
| Level 3 | Corporate debt                                                                                         |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Level 3 | Mortgage-backed securities                                                                             |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Level 3 | Municipal bonds                                                                                        |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Other special use funds                                                                                               | 0             | 0             |
| Level 3 | Other fixed income                                                                                     |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | 0             | 0             |
| Fair Value Measured at Net Asset Value Per Share | U.S. commingled equity funds                                       |          |          |
| **Assets**                                                                                                            |          |          |
| Nuclear decommissioning trust                                                                                         | $ 567,950     | $ 595,048     |

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| - Definition
Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
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| - Definition
The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning facilities, whether such amount is presented as a separate caption or as a parenthetical disclosure on the balance sheet. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.
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| - Definition
Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.
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| - Definition
Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.
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| - Definition
Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.
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| - Definition
Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.
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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.
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