Financing: At-the-Market Equity Offering Program (Details) - Common Stock - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Proceeds, (net of issuance costs)	$ 108.6	$ 52.8	$ 181.6	$ 107.7
Average price per share	$ 56.84	$ 57.33	$ 55.63	$ 60.02
Number of shares issued	1.9	0.9	3.3	1.8
Payments of Stock Issuance Costs	$ (1.1)	$ (0.5)	$ (1.8)	$ (1.1)
August 4, 2020 ATM Program
Proceeds, (net of issuance costs)	$ 0.0	$ 0.0	$ 0.0	$ 48.5
Number of shares issued	0.0	0.0	0.0	0.8
Payments of Stock Issuance Costs	$ 0.0	$ 0.0	$ 0.0	$ (0.5)
June 16, 2023 ATM Program
Proceeds, (net of issuance costs)	$ 108.6	$ 52.8	$ 181.6	$ 59.2
Number of shares issued	1.9	0.9	3.3	1.0
Payments of Stock Issuance Costs	$ (1.1)	$ (0.5)	$ (1.8)	$ (0.6)