Pension and Postretirement Benefits - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss)
Net actuarial gain (loss), tax expense (benefit)	$ (1.1)	$ (15.6)	$ 38.3
Prior service credit (cost), tax benefit (expense)	(0.2)	0.1	(0.1)
Amortization of actuarial (loss) gain, tax expense (benefit) (less than)	(0.7)	0.1	0.6
Amortization of prior service (cost) credit, tax (expense) benefit (less than)	(0.1)	(0.1)	(0.1)
Pension plans
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss)
Actuarial (loss) gain arising during the year, before tax benefit (expense)	(3.6)	(61.9)	145.1
Prior service credit (cost) arising during the year, before tax benefit (expense)	0.5	0.2	0.3
Amortization of actuarial (loss) gain, before tax benefit (expense)	2.4	(0.1)	(2.3)
Amortization of prior service (cost) credit, before tax benefit (expense)	0.0	0.0	0.0
Postretirement benefit obligations
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss)
Actuarial (loss) gain arising during the year, before tax benefit (expense)	0.0	0.2	0.1
Prior service credit (cost) arising during the year, before tax benefit (expense)	0.0	(0.1)	0.0
Amortization of actuarial (loss) gain, before tax benefit (expense)	0.0	0.0	0.0
Amortization of prior service (cost) credit, before tax benefit (expense)	$ 0.4	$ 0.4	$ 0.4