Intangible Assets - Gross and Net Intangible Assets by Major Class (Details) - USD ($) $ in Millions	Jun. 30, 2023	Sep. 30, 2022
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross amount	$ 71.0	$ 61.5
Finite-lived intangible assets, accumulated amortization	(50.9)	(39.0)
Total intangible assets subject to amortization	20.1	22.5
Indefinite-lived intangible assets	5.7	5.5
Intangible assets, gross amount	76.7	67.0
Total intangible assets, net amount	25.8	28.0
Website domains
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived license agreements	2.0	1.8
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	4.1	3.7
Finite-lived intangible assets, accumulated amortization	(2.2)	(1.6)
Total intangible assets subject to amortization	1.9	2.1
Software programs/platforms
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived software programs/platforms, gross amount	28.6	28.3
Finite-lived intangible assets, accumulated amortization	(26.1)	(19.4)
Total intangible assets subject to amortization	2.5	8.9
Client and supplier base
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived customer base, gross amount	38.3	29.5
Finite-lived intangible assets, accumulated amortization	(22.6)	(18.0)
Total intangible assets subject to amortization	$ 15.7	$ 11.5