Financing (Total Indebtedness to Capitalization) (Details) $ in Millions	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Debt Disclosure [Abstract]
Short-term borrowings	$ 4.6	$ 299.7
Long-term debt	640.3	545.1
Total debt	644.9
Equity	964.0	$ 783.8	$ 763.8	$ 914.1
Capitalization	$ 1,608.9
Total indebtedness to capitalization	0.401