Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Loss	Treasury Stock
Balance at Sep. 30, 2017	$ 696.2		$ 0.7	$ 894.0	$ (178.0)		$ (20.5)
Balance, Shares at Sep. 30, 2017			77.1
Net income (loss)	(15.1)				(15.1)
Other comprehensive income (loss), net of tax	13.6						13.6
Capital contributions and issuance of common stock	506.7		$ 0.3	506.4
Capital contributions and issuance of common stock, Shares			27.5
Equity-based compensation	28.8			28.8
Repurchase of common stock and distributions	(2.9)			(2.9)
Repurchase of common stock and distributions, Shares			(0.1)
Reclassification of effects of tax reform enactment					3.5		(3.5)
Balance at Sep. 30, 2018	1,227.3		$ 1.0	1,426.3	(189.6)		(10.4)
Balance, Shares at Sep. 30, 2018			104.5
Net income (loss)	44.4				44.4
Other comprehensive income (loss), net of tax	(1.3)						(1.3)
Capital contributions and issuance of common stock, Shares			0.2
Equity-based compensation	15.7			15.7
Repurchase of common stock and distributions	$ (1.2)			(0.2)				$ (1.0)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201409Member					us-gaap:AccountingStandardsUpdate201409Member
Balance at Sep. 30, 2019	$ 1,283.8	$ (1.1)	$ 1.0	1,441.8	(146.3)	$ (1.1)	(11.7)	(1.0)
Balance, Shares at Sep. 30, 2019			104.7
Net income (loss)	(41.6)				(41.6)
Other comprehensive income (loss), net of tax	4.8						4.8
Capital contributions and issuance of common stock	2.4			2.4
Capital contributions and issuance of common stock, Shares			0.2
Equity-based compensation	23.6			23.6
Repurchase of common stock and distributions	(1.5)							(1.5)
Balance at Sep. 30, 2020	$ 1,271.5		$ 1.0	$ 1,467.8	$ (187.9)		$ (6.9)	$ (2.5)
Balance, Shares at Sep. 30, 2020			104.9