Income Taxes - Effective Income Tax Rate Varied from Statutory Guernsey Tax Rate (Detail)	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Schedule of Effective Income Tax Rate Reconciliation [Line Items]
Foreign taxes (percentage)	15.60%	15.90%	14.80%
Effective income tax rate	15.60%	15.90%	14.80%
GUERNSEY
Schedule of Effective Income Tax Rate Reconciliation [Line Items]
Statutory Guernsey tax rate	0.00%	0.00%	0.00%

Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to statutory income tax expense (benefit) outside of the country of domicile.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I)
-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

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Data Type:	num:percentItemType
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Period Type:	duration