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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, 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%

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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13531-108611 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13537-108611 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592 + Details
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