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Fair Value of Financial Instruments - Summary of Changes in Fair Value of Investments (Detail) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Investments, at fair value |  | $ 16.8 | $ 14.4
Energy Marketing Contract Liabilities, Current |  | 0.5 | 0.0
Energy Marketing Contract Liabilities, Total | [1] | 1.8 | 0.1
Energy Marketing Contracts Assets, Total | [2] | 0.1 | 0.7
Quoted Prices in Active Markets (Level 1) [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Assets, Fair Value Disclosure |  | 15.9 | 13.3
Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Assets, Fair Value Disclosure |  | 0.1 | 0.7
Significant Unobservable Inputs (Level 3) [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Assets, Fair Value Disclosure |  | 0.9 | 1.1
Recurring [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Assets, Fair Value Disclosure |  | 16.9 | 15.1
Energy Marketing Contract Liabilities, Current |  |  | 0.1
Investments in guaranteed income fund [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Investments, at fair value |  | 0.9 | 1.1
Investments in guaranteed income fund [Member] | Recurring [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Investments, at fair value |  | 0.9 | 1.1
Investments - Mutual funds and other [Member] | Quoted Prices in Active Markets (Level 1) [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Investments, at fair value |  | 15.9 | 13.3
Investments - Mutual funds and other [Member] | Recurring [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Investments, at fair value |  |  | 13.3
Total Investments [Member] | Quoted Prices in Active Markets (Level 1) [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Investments, at fair value |  | 15.9 | 13.3
Total Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Investments, at fair value |  | 0.9 | 1.1
Total Investments [Member] | Recurring [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Investments, at fair value |  | 16.8 | 14.4
Mark To Market Energy Assets Including Put Option [Member] | Recurring [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Derivative Asset |  | $ 0.1 | $ 0.7
[1] | (1) Derivative liabilities, at fair value, include $0.5 million in current liabilities in the condensed consolidated balance sheet at September 30, 2025 and no current derivative liabilities at December 31, 2024, with the remainder of the balances classified as long-term.
[2] | (1) Derivative assets, at fair value, include $0.1 million in current assets in the condensed consolidated balance sheet at September 30, 2025 and $0.6 million at December 31, 2024, with the remainder of the balances classified as long-term

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- Definition Fair value, after the 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. 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