REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions |
Sep. 30, 2026 |
Dec. 31, 2025 |
|---|---|---|
| Regulatory assets | ||
| Regulatory assets | $ 454.6 | $ 446.2 |
| Environmental remediation costs | ||
| Regulatory assets | ||
| Regulatory assets | 126.6 | 133.5 |
| Plant retirement related items | ||
| Regulatory assets | ||
| Regulatory assets | 110.0 | 122.9 |
| AROs | ||
| Regulatory assets | ||
| Regulatory assets | 66.4 | 49.1 |
| Income tax related items | ||
| Regulatory assets | ||
| Regulatory assets | 62.9 | 59.3 |
| Electric transmission costs | ||
| Regulatory assets | ||
| Regulatory assets | 29.0 | 15.4 |
| Finance and operating leases | ||
| Regulatory assets | ||
| Regulatory assets | 12.5 | 10.2 |
| Derivatives | ||
| Regulatory assets | ||
| Regulatory assets | 11.0 | 4.8 |
| Revenue requirements of generation facilities | ||
| Regulatory assets | ||
| Regulatory assets | 8.3 | 0.6 |
| Pension and OPEB costs | ||
| Regulatory assets | ||
| Regulatory assets | 8.0 | 19.8 |
| Bluewater Natural Gas Holding, LLC | ||
| Regulatory assets | ||
| Regulatory assets | 5.0 | 8.7 |
| Other, net | ||
| Regulatory assets | ||
| Regulatory assets | $ 14.9 | $ 21.9 |