Financing: Summary of ATM Activity (Details) - Common Stock - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Proceeds, (net of issuance costs)	$ 118.7	$ 90.3	$ 118.8
Number of shares issued	2	1.3	1.8
Average price per share	$ 59.04	$ 69.74	$ 66.18
Payments of Stock Issuance Costs	$ (1.2)	$ (0.9)	$ (1.1)
August 4, 2020 ATM Program
Proceeds, (net of issuance costs)	$ 48.5	$ 90.3	$ 118.8
Number of shares issued	0.8	1.3	1.8
Payments of Stock Issuance Costs	$ (0.5)	$ (0.9)	$ (1.1)
June 16, 2023 ATM Program
Proceeds, (net of issuance costs)	$ 70.2	$ 0.0	$ 0.0
Number of shares issued	1.2	0	0
Payments of Stock Issuance Costs	$ (0.7)	$ 0.0	$ 0.0