Intangible Assets - Gross and Net Intangible Assets by Major Class (Details) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross amount	$ 72.8	$ 72.8
Finite-lived intangible assets, accumulated amortization	(43.2)	(35.9)
Total intangible assets subject to amortization	29.6	36.9
Indefinite-lived intangible assets	5.8	5.8
Intangible assets, gross amount	78.6	78.6
Total intangible assets, net amount	35.4	42.7
Website domains
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived license agreements	2.1	2.1
Business licenses
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets	3.7	3.7
Trade/domain names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived trade/domain names, gross amount	3.7	3.7
Finite-lived intangible assets, accumulated amortization	(1.2)	(0.9)
Total intangible assets subject to amortization	2.5	2.8
Software programs/platforms
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived software programs/platforms, gross amount	31.4	31.4
Finite-lived intangible assets, accumulated amortization	(18.0)	(13.3)
Total intangible assets subject to amortization	13.4	18.1
Customer base
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived customer base, gross amount	37.7	37.7
Finite-lived intangible assets, accumulated amortization	(24.0)	(21.7)
Total intangible assets subject to amortization	$ 13.7	$ 16.0