Earnings (Loss) Per Share of Common Stock - Reconciliation of Numerator and Denominator for Basic and Diluted Earnings (loss) Per Share Calculation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net income (loss) available to common stockholders	$ 25.2	$ (2.4)	$ 19.5	$ (35.5)
Weighted average number of common shares outstanding – basic	105,202,000	103,793,000	105,177,000	103,583,000
Basic earnings (loss) per share	$ 0.24	$ (0.02)	$ 0.19	$ (0.34)
Weighted average number of common shares outstanding – diluted	105,947,000	103,793,000	105,662,000	103,583,000
Diluted earnings (loss) per share	$ 0.24	$ (0.02)	$ 0.18	$ (0.34)
Weighted average number of anti-dilutive options and restricted stock	6,897,000	8,760,000	7,468,000	8,285,000