Intangible Assets - Gross and Net Intangible Assets by Major Class (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross amount	$ 72.8	$ 70.9
Finite-lived intangible assets, accumulated amortization	(35.9)	(21.4)
Finite-lived intangible assets, net amount	36.9	49.5
Indefinite-lived intangible assets	5.8	5.3
Intangible assets, gross amount	78.6	76.2
Total intangible assets, net amount	42.7	54.8
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	3.2
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	(0.9)	(0.2)
Finite-lived intangible assets, net amount	2.8	3.5
Software programs/platforms
Acquired Finite-Lived Intangible Assets [Line Items]
Capitalized software development	31.4	29.0
Finite-lived intangible assets, accumulated amortization	(13.3)	(4.9)
Finite-lived intangible assets, net amount	18.1	24.1
Customer base
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived customer base, gross amount	37.7	38.2
Finite-lived intangible assets, accumulated amortization	(21.7)	(16.3)
Finite-lived intangible assets, net amount	$ 16.0	$ 21.9