Stockholders' Equity - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 203.2	$ 306.6
Balance at end of period	185.6	203.2
Unamortized benefit plan credits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	13.4	15.1
Other comprehensive income before reclassification (loss)	0.0	0.0
(Gain) loss reclassified out of accumulated other comprehensive income	(2.1)	(2.2)
Tax benefit (expense)	0.4	0.5
Balance at end of period	11.7	13.4
Currency translation adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(6.0)	(49.7)
Other comprehensive income before reclassification (loss)	(0.4)	13.1
(Gain) loss reclassified out of accumulated other comprehensive income	4.4	30.7
Tax benefit (expense)	0.2	(0.1)
Balance at end of period	(1.8)	(6.0)
Changes in fair value of cash flow hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	5.8	13.3
Other comprehensive income before reclassification (loss)	(10.0)	(22.8)
(Gain) loss reclassified out of accumulated other comprehensive income	2.3	12.7
Tax benefit (expense)	1.9	2.6
Balance at end of period	0.0	5.8
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	13.2	(21.3)
Other comprehensive income before reclassification (loss)	(10.4)	(9.7)
(Gain) loss reclassified out of accumulated other comprehensive income	4.6	41.2
Tax benefit (expense)	2.5	3.0
Balance at end of period	$ 9.9	$ 13.2