Acquisitions (Details) $ in Thousands			3 Months Ended		9 Months Ended
	Dec. 31, 2020 USD ($) Location	Dec. 01, 2020 USD ($) Location	Jun. 30, 2021 USD ($) Store	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) Store	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)
Acquisition [Abstract]
Number of stores | Store			69		69
Payment of contingent consideration					$ 0	$ 1,457
Revenue			$ 404,207	$ 408,273	947,901	751,934
Income before income tax expense			63,055	45,359	114,482	47,743
Costs related to acquisition			65	31	633	3,393
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Goodwill			$ 151,564		151,564		$ 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]
Accounts receivable		$ 109
Inventories		5,326
Prepaid expenses		18
Property and equipment		341
Identifiable intangible assets		2,940
Goodwill		6,854
Accrued expenses		(3)
Customer deposits		(1,322)
Notes payable - floor plan		(4,016)
Total purchase price		$ 10,247
Walker Marine Group [Member]
Acquisition [Abstract]
Number of stores | Location	5
Cash paid for acquisition	$ 29,700
Payment of contingent consideration	5,500
Minimum payout due of contingent consideration	200
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Accounts receivable	129
Inventories	8,481
Prepaid expenses	39
Property and equipment	503
Identifiable intangible assets	8,230
Goodwill	28,658
Accounts payable	(213)
Customer deposits	(3,033)
Notes payable - floor plan	(7,563)
Total purchase price	$ 35,231
Roscioli Yachting Center [Member]
Acquisition [Abstract]
Number of stores | Location	1
Revenue			$ 42,700		75,500
Income before income tax expense			6,800		10,300
Costs related to acquisition			$ 100		600
Summary of Assets Acquired and Liabilities Assumed [Abstract]
Inventories	$ 87
Prepaid expenses	1
Property and equipment	41,300
Identifiable intangible assets	1,530
Goodwill	2,993
Accounts payable	(180)
Accrued expenses	(185)
Total purchase price	$ 45,546
Business Acquisition, Pro Forma Information [Abstract]
Business Acquisition, Pro Forma Revenue				440,359	990,150	844,878
Business Acquisition, Pro Forma Net Income (Loss)				$ 43,670	$ 99,099	$ 48,336