Debt and Credit Arrangements (Schedule of Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Total principal debt	$ 786.9	$ 786.9
Unamortized debt issuance costs	(3.7)	(4.5)
Total	783.2	782.4
Senior debentures maturing April 15, 2026 [Member]
Debt Instrument [Line Items]
Total principal debt	375.0	375.0
Senior debentures maturing September 1, 2030 [Member]
Debt Instrument [Line Items]
Total principal debt	300.0	300.0
Senior debentures maturing October 15, 2025 [Member]
Debt Instrument [Line Items]
Total principal debt	61.8	61.8
Subordinated debentures maturing February 3, 2027 [Member]
Debt Instrument [Line Items]
Total principal debt	$ 50.1	$ 50.1