Acquisitions, Other Acquisitions (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2021 USD ($) Store	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Aug. 01, 2021 Store	Dec. 01, 2020 Location	Aug. 01, 2019 Store	May 01, 2019 Store	Feb. 01, 2019 Store	Dec. 01, 2018 Store
Acquisition [Abstract]
Number of stores | Store	70
Payment of contingent consideration	$ 0	$ 1,456	$ 0
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Goodwill	168,491	113,059	113,059
Revenue	1,228,206	1,022,970
Income before income tax expense	142,215	54,837
Net income	79,059	17,425
Costs related to acquisition	869	$ 3,648	1,300
Tom George Yacht Group [Member]
Acquisition [Abstract]
Number of stores | Location					2
Stone Harbor Marina [Member]
Acquisition [Abstract]
Number of stores | Store				1
Slalom Shop [Member]
Acquisition [Abstract]
Number of stores | Store									2
Ocean Blue [Member]
Acquisition [Abstract]
Number of stores | Store								3
Ray Clepper [Member]
Acquisition [Abstract]
Number of stores | Store								1
Caribee [Member]
Acquisition [Abstract]
Number of stores | Store							1
Central Marine [Member]
Acquisition [Abstract]
Number of stores | Store						3
Retail Boat Dealer Groups [Member]
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Revenue			62,000
Net income			4,000
Slalom Shop, Ocean Blue, Ray Clepper, Caribee and Central Marine [Member]
Acquisition [Abstract]
Cash paid for acquisition			19,400
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Total purchase price			48,600
Slalom Shop, Ocean Blue, Ray Clepper, Caribee and Central Marine [Member] | Long-term Debt [Member]
Acquisition [Abstract]
Note payable to seller			$ 29,200
Tom George Yacht Group, Stone Harbor Marina and PartsVu [Member]
Acquisition [Abstract]
Cash paid for acquisition	32,200
Note payable to seller	2,100
Payment of contingent consideration	5,100
Accrued purchase consideration	1,900
Issuance of shares of Class A common stock	1,500
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Accounts receivable	390
Inventories	10,476
Prepaid expenses	180
Property and equipment	700
Identifiable intangible assets	13,940
Goodwill	25,512
Accrued expenses	(47)
Customer deposits	(2,248)
Notes payable - floor plan	(6,134)
Total purchase price	42,769
Revenue	107,900
Income before income tax expense	13,300
Costs related to acquisition	$ 800