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, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement components	$ (0.2)		$ 1.0		$ 0.4	$ 2.4
Total before tax	55.4		55.9		102.1	110.5
Tax impact	13.8		12.7		22.7	23.6
Net income attributable to common shareholders	41.6	$ 37.8	43.2	$ 43.7	79.4	86.9
Reclassification out of Accumulated Other Comprehensive Income | Pension and Postretirement Benefits
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Total before tax	(0.6)		(0.6)		(1.1)	(1.2)
Tax impact	(0.1)		(0.1)		(0.2)	(0.2)
Net income attributable to common shareholders	(0.5)		(0.5)		(0.9)	(1.0)
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 components	(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 components	0.0		0.0		0.1	0.0
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 components	(0.2)		(0.2)		(0.4)	(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 components	$ (0.2)		$ (0.2)		$ (0.4)	$ (0.3)