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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
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
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 | 70.20% | 70.10% | 71.20%
Used Boat Sales [Member] |  |  |
Product Information [Line Items] |  |  |
Sales Revenue Goods And Services Net Percentage | 15.10% | 14.90% | 14.80%
Maintenance, Repair, Storage and Charter Services [Member] |  |  |
Product Information [Line Items] |  |  |
Sales Revenue Goods And Services Net Percentage | 6.40% | 6.90% | 6.20%
Finance and Insurance Products [Member] |  |  |
Product Information [Line Items] |  |  |
Sales Revenue Goods And Services Net Percentage | 2.70% | 2.60% | 2.40%
Parts and Accessories [Member] |  |  |
Product Information [Line Items] |  |  |
Sales Revenue Goods And Services Net Percentage | 3.00% | 3.60% | 3.60%
Brokerage Sales [Member] |  |  |
Product Information [Line Items] |  |  |
Sales Revenue Goods And Services Net Percentage | 2.60% | 1.90% | 1.80%

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