Nature of Operations and Significant Accounting Policies (Amounts Reclassified out of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	$ 2.5	$ 3.1	$ 4.7
Income before income taxes	239.8	248.3	226.1
Tax impact	51.5	43.4	48.1
Net income attributable to common shareholders	188.3	204.9	178.0
Amount Reclassified from Accumulated Other Comprehensive Loss | Pension and Postretirement
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income before income taxes	(2.5)	(1.2)	(0.3)
Tax impact	(0.5)	(0.3)	(0.1)
Net income attributable to common shareholders	(2.0)	(0.9)	(0.2)
Pension | Amount Reclassified from Accumulated Other Comprehensive Loss | Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	(0.7)	(0.7)	(0.7)
Pension | Amount Reclassified from Accumulated Other Comprehensive Loss | Net gain (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	0.0	0.6	1.5
Postretirement | Amount Reclassified from Accumulated Other Comprehensive Loss | Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	(1.1)	(1.1)	(1.1)
Postretirement | Amount Reclassified from Accumulated Other Comprehensive Loss | Net gain (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	$ (0.7)	$ 0.0	$ 0.0