Changes in Accumulated Other Comprehensive Loss (Details of Accumulated Other Comprehensive Income Components) (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement items	$ 0.7			$ 0.4			$ 3.1	$ 3.3
Total before tax	67.9			64.4			178.4	189.4
Tax impact	16.0			19.8			39.6	46.5
Net income attributable to common shareholders	51.9	$ 43.2	$ 43.7	44.6	$ 48.8	$ 49.5	138.8	142.9
Reclassification out of Accumulated Other Comprehensive Income | Pension and Postretirement Plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Total before tax	(0.6)			(0.1)			(1.8)	(0.6)
Tax impact	(0.1)			(0.1)			(0.3)	(0.2)
Net income attributable to common shareholders	(0.5)			0.0			(1.5)	(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	(0.1)			(0.1)			(0.5)	(0.5)
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			0.3			0.0	0.7
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	(0.3)			(0.3)			(0.8)	(0.8)
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			$ (0.5)	$ 0.0