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Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Sep. 30, 2017 USD ($) Segment	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jan. 31, 2017 USD ($)	Apr. 30, 2016 USD ($)
Significant Accounting Policies [Line Items]
Inventories valuation allowance, net realizable value	$ 1,800,000	$ 1,000,000
Goodwill	25,900,000	9,900,000
Impairment charges	0
Total advertising and promotional expenses	16,200,000	13,500,000	$ 10,500,000
Net of related co-op assistance	$ 779,000	$ 730,000	$ 737,000
Number of reporting segment | Segment	1
Russo Marine [Member]
Significant Accounting Policies [Line Items]
Goodwill					$ 8,800,000
Hall Marine Group [Member]
Significant Accounting Policies [Line Items]
Goodwill				$ 16,000,000
Hurricane Irma [Member] | British Virgin Islands [Member]
Significant Accounting Policies [Line Items]
Inventory damages	$ 7,600,000

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Inventory damaged.

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Significant accounting policies.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

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The total expense recognized in the period for promotion, public relations, and brand or product advertising.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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