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, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and postretirement components	$ (1.3)			$ 0.7			$ 1.7	$ 3.1
Total before tax	63.3			67.9			165.4	178.4
Tax impact	16.2			16.0			38.9	39.6
Net income attributable to common shareholders	47.1	$ 41.6	$ 37.8	51.9	$ 43.2	$ 43.7	126.5	138.8
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	(2.0)			(0.6)			(3.1)	(1.8)
Tax impact	(0.2)			(0.1)			(0.4)	(0.3)
Net income attributable to common shareholders	(1.8)			(0.5)			(2.7)	(1.5)
Pension Plan [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other	1.4						1.4
Pension Plan [Member] | 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)			(0.1)			(0.6)	(0.5)
Pension Plan [Member] | 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			0.2	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	(0.3)			(0.3)			(0.7)	(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 components	(0.2)			$ (0.2)			(0.6)	$ (0.5)
Other	$ (1.4)						$ (1.4)