Financing: At-the-Market Equity Offering Program (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Number of shares issued	436,202	216,885	881,780	272,592
August 4, 2020 ATM Program
Number of shares issued	329,647	216,885	775,225	272,592
June 16, 2023 ATM Program
Number of shares issued	106,555		106,555
Common Stock
Proceeds, (net of issuance costs)	$ 27.4	$ 16.4	$ 54.9	$ 20.2
Average price per share	$ 63.53	$ 76.39	$ 62.86	$ 74.84
Payments of Stock Issuance Costs	$ (0.3)	$ (0.2)	$ (0.6)	$ (0.2)
Common Stock \| August 4, 2020 ATM Program
Proceeds, (net of issuance costs)	21.0	16.4	48.5	20.2
Payments of Stock Issuance Costs	(0.2)	(0.2)	(0.5)	(0.2)
Common Stock \| June 16, 2023 ATM Program
Proceeds, (net of issuance costs)	6.4		6.4
Payments of Stock Issuance Costs	$ (0.1)	$ 0.0	$ (0.1)	$ 0.0