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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, 2017	Sep. 30, 2016	Sep. 30, 2015
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	70.90%	68.50%	64.30%
Used Boat Sales [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	14.90%	17.50%	19.90%
Maintenance, Repair, Storage and Charter Services [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	6.30%	6.00%	6.90%
Finance and Insurance Products [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	2.40%	2.50%	2.50%
Parts and Accessories [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	3.60%	3.50%	4.10%
Brokerage Sales [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	1.90%	2.00%	2.30%

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

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- Definition

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