Segment Information - Property and Equipment Net on Geographical Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Property and equipment, net	$ 48,837	$ 40,703
United States
Segment Reporting Information [Line Items]
Property and equipment, net	39,593	30,773
United Kingdom
Segment Reporting Information [Line Items]
Property and equipment, net	4,296	4,893
Other countries
Segment Reporting Information [Line Items]
Property and equipment, net	$ 4,948	$ 5,037
Geographic Concentration Risk \| Property Plant and Equipment
Segment Reporting Information [Line Items]
Concentration of risk, Percentage	100.00%	100.00%
Geographic Concentration Risk \| United States \| Property Plant and Equipment
Segment Reporting Information [Line Items]
Concentration of risk, Percentage	81.00%	76.00%
Geographic Concentration Risk \| United Kingdom \| Property Plant and Equipment
Segment Reporting Information [Line Items]
Concentration of risk, Percentage	9.00%	12.00%
Geographic Concentration Risk \| Other countries \| Property Plant and Equipment
Segment Reporting Information [Line Items]
Concentration of risk, Percentage	10.00%	12.00%

Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(14))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(8))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(13))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_PropertyPlantAndEquipmentNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant