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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, 2018	Sep. 30, 2017	Sep. 30, 2016
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	71.20%	70.90%	68.50%
Used Boat Sales [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	14.80%	14.90%	17.50%
Maintenance, Repair, Storage and Charter Services [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	6.20%	6.30%	6.00%
Finance and Insurance Products [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	2.40%	2.40%	2.50%
Parts and Accessories [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	3.60%	3.60%	3.50%
Brokerage Sales [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	1.80%	1.90%	2.00%

X

- 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
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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No definition available.

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