Significant Accounting Policies - Segment Reporting Revenue Percentage (Detail) - Sales Revenue, Segment - Revenue from Rights Concentration Risk	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Reinforcement Materials [Member]
Segment Reporting Information [Line Items]
Revenue, by segment	57.00%	53.00%	48.00%
Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Revenue, by segment	33.00%	35.00%	37.00%
Purification Solutions [Member]
Segment Reporting Information [Line Items]
Revenue, by segment	9.00%	11.00%	13.00%
Specialty Fluids [Member]
Segment Reporting Information [Line Items]
Revenue, by segment	1.00%	1.00%	2.00%

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

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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-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13531-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611

Reference 4: 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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