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Income Taxes - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Income Taxes [Line Items]        
Net decrease in valuation allowances $ 3,905 $ 16,078    
Tax credits, net capital and operating loss carryforwards 756,414 829,260    
Tax credits, net capital and operating loss carryforwards having expiration date $ 170,430 $ 245,843    
Tax Credit net capital and operating loss Carryforwards Expiration Dates Through 2041 for $170,430 tax credit, net capital and operating loss carryforwards Through 2040 for $245,843 tax credit, net capital and operating loss carryforwards    
Gross unrecognized tax benefits $ 195,196 $ 168,186 $ 169,322 $ 187,646
Accrued income taxes payable for interest and penalties relating to unrecognized tax benefits 25,011 15,812    
Tax benefit recognized in the statements of income, net of interest and penalty reversals 9,199 15,396    
Tax benefit related to the release of withholding and income tax reserves for unremitting earnings 28,795 $ 28,795    
Maximum [Member]        
Income Taxes [Line Items]        
Expected decrease in unrecognized tax benefits within next 12 months due to lapse of statutes of limitations $ 3,559      
GUERNSEY        
Income Taxes [Line Items]        
Statutory Guernsey tax rate 0.00% 0.00% 0.00%  
Prepaid Expenses and Other Current Assets [Member]        
Income Taxes [Line Items]        
Tax receivable balance $ 53,925 $ 58,336