Recent Accounting Pronouncements - Additional Informaiton (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2020	Oct. 01, 2019	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncement Early Adoption [Line Items]
Cumulative effect on retained earnings	$ 1,283.8	$ 1,271.5		$ 1,227.3	$ 696.2
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201409Member
Right-of-use asset and lease liability classified as deferred rent obligations and exit/disposal liabilities, and prepaid rent			$ 5.7
Accounting Standards Update 2016-02
New Accounting Pronouncement Early Adoption [Line Items]
Right-of-use asset			70.6
Lease liability			$ 76.3
Accumulated Deficit
New Accounting Pronouncement Early Adoption [Line Items]
Cumulative effect on retained earnings	$ (146.3)	$ (187.9)		$ (189.6)	$ (178.0)
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncement Early Adoption [Line Items]
Cumulative effect on retained earnings	(1.1)
Cumulative Effect, Period of Adoption, Adjustment | Accumulated Deficit
New Accounting Pronouncement Early Adoption [Line Items]
Cumulative effect on retained earnings	$ (1.1)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201409Member