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Income Taxes - Effective Income Tax Rate Varied from Statutory Guernsey Tax Rate (Parenthetical) (Detail) - USD ($)
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Schedule Of Effective Income Tax Rate Reconciliation [Line Items]      
Releases of gross unrecognized tax benefits related to settled arrangements with tax authorities, change in measurement of certain positions and expiration of periods set forth in statutes of limitations $ 40,644,000 $ 4,757,000 $ 7,701,000
Tax (expense) benefit resulting from the release of valuation allowances on deferred tax assets   1,211,000 6,006,000
Foreign tax benefit from internal structural changes     10,933,000
Foreign tax expense for the estimated additional tax charge of gain from sale of a business   3,193,000 $ 39,596,000
Release of gross unrecognized tax benefits offset by a net benefit included within income tax expense 16,232,000    
Tax benefit change in measurement of a deferred tax liability 9,236    
Tax Benefit Internal Structural Changes In Certain Jurisdictions 11,566,000    
Benefit of relating to changes in tax regulations in certain jurisdiction 3,142,000 8,871,000  
ISRAEL      
Schedule Of Effective Income Tax Rate Reconciliation [Line Items]      
Tax benefit related to the release of accrued withholding taxes on unremitted earnings $ 22,700,000 $ 37,000,000