Intangible Assets - Gross and Net Intangible Assets by Major Class (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross amount	$ 70.9	$ 70.9
Finite-lived intangible assets, accumulated amortization	(25.1)	(21.4)
Total intangible assets subject to amortization	45.8	49.5
Indefinite-lived intangible assets, gross amount	5.8	5.3
Intangible assets, gross amount	76.7	76.2
Total intangible assets, net amount	51.6	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, gross amount	3.7	3.2
Trade/domain names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived trade names, gross amount	3.7	3.7
Finite-lived intangible assets, accumulated amortization	(0.3)	(0.2)
Total intangible assets subject to amortization	3.4	3.5
Software programs/platforms
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived computer software, gross amount	29.0	29.0
Finite-lived intangible assets, accumulated amortization	(7.1)	(4.9)
Total intangible assets subject to amortization	21.9	24.1
Customer base
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived customer lists, gross amount	38.2	38.2
Finite-lived intangible assets, accumulated amortization	(17.7)	(16.3)
Total intangible assets subject to amortization	$ 20.5	$ 21.9