Risk Management and Derivatives: Derivatives Not Designated as Hedge Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Derivative [Line Items]
Regulatory assets	$ 480.1	$ 653.0
Gas price derivatives
Derivative [Line Items]
Regulatory assets	5.1	8.8
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ (4.2)	$ (0.8)	$ 2.6
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
Not Designated as Hedging Instrument | Natural Gas
Derivative [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ (4.2)	$ (0.8)	$ 2.6
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