Acquisitions (Details) $ in Thousands			3 Months Ended		6 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 01, 2020 USD ($) Location	Mar. 31, 2021 USD ($) Store Location	Mar. 31, 2020 USD ($)	Mar. 31, 2021 USD ($) Store Location	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Acquisition [Abstract]
Number of stores | Store			69		69
Payment of contingent consideration					$ 0	$ 1,457
Revenue			$ 329,611	$ 189,963	543,694	343,661
Income before income tax expense			37,142	3,451	51,428	2,384
Costs related to acquisition			368	2,925	568	3,362
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Goodwill			$ 151,417		$ 151,417		$ 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
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	$ 31,800
Payment of contingent consideration	5,500
Minimum payout due of contingent consideration	200
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Tangible assets	503
Identifiable intangible assets	8,230
Goodwill	28,511
Total purchase price	37,244
Roscioli Yachting Center [Member]
Acquisition [Abstract]
Number of stores | Location			1		1
Revenue			$ 30,700		$ 32,800
Income before income tax expense			3,300		3,400
Costs related to acquisition			$ 400		600
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Tangible assets	41,300
Other tangible assets	88
Identifiable intangible assets	1,530
Goodwill	2,993
Liabilities assumed	(365)
Total purchase price	$ 45,546
Business Acquisition, Pro Forma Information [Abstract]
Business Acquisition, Pro Forma Revenue				218,884	585,943	404,519
Business Acquisition, Pro Forma Net Income (Loss)				$ 5,653	$ 47,542	$ 4,666