Changes in Accumulated Other Comprehensive Loss (Details of Accumulated Other Comprehensive Income Components) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement components	$ 0.6	$ 1.4
Total before tax	46.7	54.6
Tax impact	8.9	10.9
Net income attributable to common shareholders	37.8	43.7
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	(0.5)	(0.6)
Tax impact	(0.1)	(0.1)
Net income attributable to common shareholders	(0.4)	(0.5)
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)
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	0.1	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)	(0.3)
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)	$ (0.1)