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Significant Accounting Policies - Summary of Percentages of Revenue Generated by Products and Services (Detail) - Product Concentration Risk [Member] - Sales [Member]
12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Product Information [Line Items]      
Sales Revenue Goods And Services Net Percentage 100.00% 100.00% 100.00%
New Boat Sales [Member]      
Product Information [Line Items]      
Sales Revenue Goods And Services Net Percentage 70.10% 71.20% 70.90%
Used Boat Sales [Member]      
Product Information [Line Items]      
Sales Revenue Goods And Services Net Percentage 14.90% 14.80% 14.90%
Maintenance, Repair, Storage and Charter Services [Member]      
Product Information [Line Items]      
Sales Revenue Goods And Services Net Percentage 6.90% 6.20% 6.30%
Finance and Insurance Products [Member]      
Product Information [Line Items]      
Sales Revenue Goods And Services Net Percentage 2.60% 2.40% 2.40%
Parts and Accessories [Member]      
Product Information [Line Items]      
Sales Revenue Goods And Services Net Percentage 3.60% 3.60% 3.60%
Brokerage Sales [Member]      
Product Information [Line Items]      
Sales Revenue Goods And Services Net Percentage 1.90% 1.80% 1.90%