Intangible Assets - Gross and Net Intangible Assets by Major Class (Details) - USD ($) $ in Millions	Dec. 31, 2023	Sep. 30, 2023
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross amount	$ 43.1	$ 70.9
Finite-lived intangible assets, accumulated amortization	(27.6)	(53.4)
Total intangible assets subject to amortization	15.5	17.5
Indefinite-lived intangible assets	5.7	5.6
Intangible assets, gross amount	48.8	76.5
Total intangible assets, net amount	21.2	23.1
Website domains
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived license agreements	2.0	1.9
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	4.1
Finite-lived intangible assets, accumulated amortization	(2.6)	(2.4)
Total intangible assets subject to amortization	1.5	1.7
Software programs/platforms
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived software programs/platforms, gross amount	4.9	28.5
Finite-lived intangible assets, accumulated amortization	(3.5)	(26.9)
Total intangible assets subject to amortization	1.4	1.6
Client and supplier base
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived customer base, gross amount	34.1	38.3
Finite-lived intangible assets, accumulated amortization	(21.5)	(24.1)
Total intangible assets subject to amortization	$ 12.6	$ 14.2