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Income Taxes - Effective Income Tax Rate Varied from Statutory Guernsey Tax Rate (Parenthetical) (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
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 $ 7,701 $ 47,582  
Tax benefit primarily attributable to expiration of periods set forth in statutes of limitations and conclusions of tax audits related to unrecognized tax benefits     $ 26,529
Tax benefit resulting from the release of valuation allowances on deferred tax assets 6,006 15,438 $ 12,650
Foreign tax benefit from internal structural changes 10,933    
Foreign tax expense for the estimated additional tax charge of gain from sale of a business $ 39,596    
Release of gross unrecognized tax benefits offset by a net benefit included within income tax expense   14,971  
Foreign tax benefit resulting from tax enactments related to COVID-19   $ 4,964