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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, 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%

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

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