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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13531-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Reference 5: 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

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Summary of significant accounting policies - Schedule of concentration of risk (Detail) - Customer Concentration Risk [Member]	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Customer A [Member] \| Sales Revenue, Net [Member]
Concentration Risk, Percentage	12.00%	17.00%	34.00%
Customer A [Member] \| Accounts Receivable [Member]
Concentration Risk, Percentage		13.00%
Customer B [Member] \| Sales Revenue, Net [Member]
Concentration Risk, Percentage	10.00%
Customer B [Member] \| Accounts Receivable [Member]
Concentration Risk, Percentage	36.00%