Changes in Accumulated Other Comprehensive Loss (Details of Accumulated Other Comprehensive Income Components) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	$ 1.0		$ 2.1		$ 2.4	$ 2.9
Income before income taxes	55.9		61.5		110.5	125.0
Tax impact	12.7		12.7		23.6	26.7
Net income attributable to common shareholders	43.2	$ 43.7	48.8	$ 49.5	86.9	98.3
Reclassification out of Accumulated Other Comprehensive Income | Pension and Postretirement Plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income before income taxes	(0.6)		(0.2)		(1.2)	(0.5)
Tax impact	(0.1)		0.0		(0.2)	(0.1)
Net income attributable to common shareholders	(0.5)		(0.2)		(1.0)	(0.4)
Pension | Reclassification out of Accumulated Other Comprehensive Income | Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	(0.2)		(0.2)		(0.4)	(0.4)
Pension | Reclassification out of Accumulated Other Comprehensive Income | Net gain (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	0.0		0.2		0.0	0.4
Postretirement | Reclassification out of Accumulated Other Comprehensive Income | Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	(0.2)		(0.2)		(0.5)	(0.5)
Postretirement | Reclassification out of Accumulated Other Comprehensive Income | Net gain (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	$ (0.2)		$ 0.0		$ (0.3)	$ 0.0