Risk Management and Derivatives: Derivatives Not Designated as Hedge Instruments (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | |||
|---|---|---|---|---|---|
Jun. 30, 2026 |
Jun. 30, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
Dec. 31, 2025 |
|
| Derivative [Line Items] | |||||
| Regulatory assets | $ 378.0 | $ 378.0 | $ 394.7 | ||
| Regulatory liabilities | 588.7 | 588.7 | 588.2 | ||
| Gas price derivatives | |||||
| Derivative [Line Items] | |||||
| Regulatory assets | 0.4 | 0.4 | $ 4.6 | ||
| Not Designated as Hedging Instrument | |||||
| Derivative [Line Items] | |||||
| Derivative, Gain (Loss) on Derivative, Net | $ 0.2 | $ (0.5) | $ 0.6 | $ 0.6 | |
| Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold | |
| Not Designated as Hedging Instrument | Natural Gas | |||||
| Derivative [Line Items] | |||||
| Derivative, Gain (Loss) on Derivative, Net | $ 0.2 | $ (0.5) | $ 0.6 | $ 0.6 | |
| Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold | Fuel, Purchased Power and Cost of Gas Sold | |