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Company Background - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Jun. 30, 2018 Store State	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Concentration Risk [Line Items]
Number of retail locations | Store	63
Number of states wherein retail locations are established | State	16
Product Concentration Risk [Member] | Brunswick [Member] | Sales [Member]
Concentration Risk [Line Items]
Revenue percentage from sale of boats		42.00%
Product Concentration Risk [Member] | Brunswick Sea Ray Boat [Member] | Brunswick [Member] | Sales [Member]
Concentration Risk [Line Items]
Revenue percentage from sale of boats		23.00%
Product Concentration Risk [Member] | Brunswick Boston Whaler Boats [Member] | Brunswick [Member] | Sales [Member]
Concentration Risk [Line Items]
Revenue percentage from sale of boats		17.00%
Product Concentration Risk [Member] | Brunswick Sea Ray Sport Yacht and Yacht Models [Member] | Brunswick [Member] | Sales [Member]
Concentration Risk [Line Items]
Revenue percentage from sale of boats	10.00%
Product Concentration Risk [Member] | Azimut Benetti Groups | Sales [Member]
Concentration Risk [Line Items]
Revenue percentage from sale of boats		9.00%
Supplier Concentration Risk [Member] | Brunswick [Member] | Sales [Member]
Concentration Risk [Line Items]
Revenue percentage from sale of boats		55.00%
Geographic Concentration Risk [Member] | Sales [Member] | Florida [Member]
Concentration Risk [Line Items]
Revenue percentage from sale of boats		55.00%	55.00%	53.00%

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