Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Income) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2021
Equity-based compensation plans
Stock-based compensation expense	$ 4.9	$ 32.8	$ 28.7	$ 29.8
Income tax benefit recognized for stock-based compensation	(0.8)	(4.4)	(4.2)	(4.4)
Stock-based compensation cost, net of tax	4.1	28.4	24.5	25.4
Stock options
Equity-based compensation plans
Stock-based compensation expense	0.2	0.0	0.3	1.8
Nonvested Stock Awards [Member]
Equity-based compensation plans
Stock-based compensation expense	3.5	24.9	25.7	19.3
Income tax benefit recognized for stock-based compensation	(3.1)	(2.8)	(2.5)	(2.0)
Performance Shares [Member]
Equity-based compensation plans
Stock-based compensation expense	0.5	7.0	2.6	6.1
Stock Appreciation Rights (SARs)
Equity-based compensation plans
Stock-based compensation expense	0.3	0.2	(0.8)	1.1
Restricted Stock Units (RSUs)
Equity-based compensation plans
Stock-based compensation expense	$ 0.4	$ 0.7	$ 0.9	$ 1.5