Goodwill and Intangible Assets - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Nov. 13, 2024	Nov. 14, 2023
Indefinite-Lived Intangible Assets [Line Items]
Impairment, intangible asset, indefinite-lived (excluding goodwill), extensible enumeration	Restructuring and impairment
Impairment, intangible asset, finite-lived (excluding goodwill), extensible enumeration	Restructuring and impairment
Goodwill, impairment loss, extensible enumeration	Restructuring and impairment
Restructuring and impairment	$ 145,844,000	$ 539,000	$ 147,402,000
Amortization expense	$ 8,066,000	7,842,000	13,400,000
Weighted average useful life	3 years 6 months
Goodwill	$ 258,954,000	336,602,000	336,602,000
Goodwill and Intangible Asset Impairment		0
Impairment, Intangible Asset, Statement of Income or Comprehensive Income [Extensible Enumeration]	Restructuring and impairment
Inventory Financing Facility
Indefinite-Lived Intangible Assets [Line Items]
Line of credit facility, maximum borrowing capacity				$ 595,000,000.0	$ 650,000,000.0
2023 Impairment
Indefinite-Lived Intangible Assets [Line Items]
Impairment and restructuring			147,400,000
Goodwill and Intangible Asset Impairment			147,400,000
2025 Impairment
Indefinite-Lived Intangible Assets [Line Items]
Impairment and restructuring	$ 149,700,000
Goodwill and Intangible Asset Impairment	145,800,000
Domain Names
Indefinite-Lived Intangible Assets [Line Items]
Amortization expense	617,000	637,000
Internally Developed Software
Indefinite-Lived Intangible Assets [Line Items]
Amortization expense	1,365,000	1,039,000	600,000
Dealerships
Indefinite-Lived Intangible Assets [Line Items]
Goodwill	259,000,000.0	295,300,000
Goodwill and Intangible Asset Impairment	48,000,000		6,500,000
Dealerships | 2023 Impairment
Indefinite-Lived Intangible Assets [Line Items]
Impairment and restructuring			6,500,000
Dealerships | 2025 Impairment
Indefinite-Lived Intangible Assets [Line Items]
Impairment and restructuring	50,000,000.0
Distribution
Indefinite-Lived Intangible Assets [Line Items]
Goodwill		$ 41,300,000
Goodwill and Intangible Asset Impairment	97,800,000		140,900,000
Distribution | 2023 Impairment
Indefinite-Lived Intangible Assets [Line Items]
Impairment and restructuring			$ 140,900,000
Distribution | 2025 Impairment
Indefinite-Lived Intangible Assets [Line Items]
Impairment and restructuring	$ 99,700,000