Financing: At-the-Market Equity Offering Program (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Number of shares issued	930,844		1,812,624	272,592
August 4, 2020 ATM Program
Number of shares issued			775,225	272,592
June 16, 2023 ATM Program
Number of shares issued	930,844		1,037,399
Common Stock
Proceeds, (net of issuance costs)	$ 52.8		$ 107.7	$ 20.2
Average price per share	$ 57.33		$ 60.02	$ 74.84
Payments of Stock Issuance Costs	$ (0.5)	$ 0.0	$ (1.1)	$ (0.2)
Common Stock \| August 4, 2020 ATM Program
Proceeds, (net of issuance costs)			48.5	20.2
Payments of Stock Issuance Costs	0.0	0.0	(0.5)	(0.2)
Common Stock \| June 16, 2023 ATM Program
Proceeds, (net of issuance costs)	52.8		59.2
Payments of Stock Issuance Costs	$ (0.5)	$ 0.0	$ (0.6)	$ 0.0