Summary of Significant Accounting Policies - Regulatory Assets (Detail) - USD ($) $ in Millions |
Jun. 30, 2026 |
Dec. 31, 2025 |
Jun. 30, 2025 |
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|---|---|---|---|---|---|
| Regulatory Assets [Line Items] | |||||
| Regulatory Assets | $ 103.9 | $ 116.9 | $ 99.2 | ||
| Less: Current Portion of Regulatory Assets | [1] | 64.8 | 78.1 | 52.6 | |
| Regulatory Assets – noncurrent | 39.1 | 38.8 | 46.6 | ||
| Other Deferred Charges | |||||
| Regulatory Assets [Line Items] | |||||
| Regulatory Assets | 16.7 | 12.8 | 14.7 | ||
| Environmental Matters | |||||
| Regulatory Assets [Line Items] | |||||
| Regulatory Assets | 10.5 | 10.0 | 9.4 | ||
| Retirement Benefits | |||||
| Regulatory Assets [Line Items] | |||||
| Regulatory Assets | 13.7 | 14.6 | 19.3 | ||
| Deferred Storm Charges | |||||
| Regulatory Assets [Line Items] | |||||
| Regulatory Assets | 4.5 | 6.0 | 7.0 | ||
| Income Taxes | |||||
| Regulatory Assets [Line Items] | |||||
| Regulatory Assets | 0.2 | 0.4 | 0.7 | ||
| Energy Supply and Other Rate Adjustment Mechanisms | |||||
| Regulatory Assets [Line Items] | |||||
| Regulatory Assets | $ 58.3 | $ 73.1 | $ 48.1 | ||
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