Financing: At-the-Market Equity Offering Program (Details) - Common Stock - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Proceeds, (net of issuance costs)	$ 41.8	$ 27.4	$ 73.0	$ 54.9
Average price per share	$ 55.02	$ 63.53	$ 53.92	$ 62.86
Number of shares issued	0.8	0.4	1.4	0.9
Payments of Stock Issuance Costs	$ (0.4)	$ (0.3)	$ (0.7)	$ (0.6)
August 4, 2020 ATM Program
Proceeds, (net of issuance costs)	$ 0.0	$ 21.0	$ 0.0	$ 48.5
Number of shares issued	0.0	0.3	0.0	0.8
Payments of Stock Issuance Costs	$ 0.0	$ (0.2)	$ 0.0	$ (0.5)
June 16, 2023 ATM Program
Proceeds, (net of issuance costs)	$ 41.8	$ 6.4	$ 73.0	$ 6.4
Number of shares issued	0.8	0.1	1.4	0.1
Payments of Stock Issuance Costs	$ (0.4)	$ (0.1)	$ (0.7)	$ (0.1)