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Income Taxes - Effective Income Tax Rate Varied from Statutory Guernsey Tax Rate (Detail)
12 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2020
Schedule of Effective Income Tax Rate Reconciliation [Line Items]      
Foreign taxes (percentage) 15.30% 15.50% 14.70%
Effective income tax rate 15.30% 15.50% 14.70%
GUERNSEY      
Schedule of Effective Income Tax Rate Reconciliation [Line Items]      
Statutory Guernsey tax rate 0.00% 0.00% 0.00%