Acquisitions (Details) $ in Thousands			3 Months Ended
	Dec. 31, 2020 USD ($) Store Location	Dec. 01, 2020 USD ($) Location	Dec. 31, 2020 USD ($) Store Location	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)
Acquisition [Abstract]
Number of stores | Store	69		69
Payment of contingent consideration			$ 0	$ 1,040
Accrued purchase consideration	$ 4,800		4,800		$ 5,500
Revenue			214,083	153,698
Pretax income			14,286	(1,067)
Costs related to acquisition			200	$ 437
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Goodwill	$ 146,562		$ 146,562		$ 113,059
Tom George Yacht Group [Member]
Acquisition [Abstract]
Number of stores | Location		2
Cash paid for acquisition		$ 8,200
Note payable to seller		$ 2,100
Interest rate		5.50%
Notes payable term			3 years
Revenue			$ 2,100
Pretax income			$ 200
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Tangible assets		$ 5,794
Identifiable intangible assets		2,940
Goodwill		6,854
Liabilities assumed		(5,341)
Total purchase price		$ 10,247
Walker Marine Group [Member]
Acquisition [Abstract]
Number of stores | Location	5		5
Cash paid for acquisition	$ 23,900
Payment of contingent consideration	4,800
Accrued purchase consideration	3,700		$ 3,700
Minimum payout due of contingent consideration	100		100
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Tangible assets	474		474
Identifiable intangible assets	8,230		8,230
Goodwill	23,643		23,643
Total purchase price	$ 32,347		$ 32,347
Roscioli Yachting Center [Member]
Acquisition [Abstract]
Number of stores | Location	1		1
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Tangible assets	$ 41,300		$ 41,300
Other tangible assets	88		88
Identifiable intangible assets	1,530		1,530
Goodwill	3,005		3,005
Liabilities assumed	(346)		(346)
Total purchase price	$ 45,577		45,577
Tom George Yacht Group Walker Marine Group Roscioli Yachting Center [Member]
Acquisition [Abstract]
Costs related to acquisition			$ 200