Summary of Significant Accounting Policies - Components of Exchange Gas Receivable (Detail) - USD ($) $ in Millions |
Jun. 30, 2026 |
Dec. 31, 2025 |
Jun. 30, 2025 |
|---|---|---|---|
| Receivables [Line Items] | |||
| Total Exchange Gas Receivable | $ 8.6 | $ 9.8 | $ 9.1 |
| Northern Utilities Inc | |||
| Receivables [Line Items] | |||
| Total Exchange Gas Receivable | 7.2 | 8.0 | 7.8 |
| Bangor | |||
| Receivables [Line Items] | |||
| Total Exchange Gas Receivable | 1.1 | 1.3 | 0.9 |
| Fitchburg | |||
| Receivables [Line Items] | |||
| Total Exchange Gas Receivable | $ 0.3 | $ 0.5 | $ 0.4 |