Summary of Significant Accounting Policies - Leases (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020	Oct. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset	$ 9.7	$ 11.9
Operating lease liability	$ 10.9	$ 13.2
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset			$ 14.8
Operating lease liability			$ 16.0