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Income Taxes - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2022
Income Taxes [Line Items]        
Net Increase (Decrease) in valuation allowances $ 1,700 $ (4,652)    
Tax credits, net capital and operating loss carryforwards 557,390 699,023    
Tax credits, net capital and operating loss carryforwards having expiration date $ 137,406 $ 202,193    
Tax credit net capital and operating loss carryforwards expiration dates through 2045 through 2044    
Gross unrecognized tax benefits $ 170,532 $ 151,822 $ 196,949 $ 213,031
Accrued income taxes payable for interest and penalties relating to unrecognized tax benefits 38,090 30,495    
Interest and penalties recognized in the statements of income, relating to unrecognized tax expense (benefit) 7,595 4,617    
Unrecognized deferred tax liability for undistributed earnings of foreign subsidiary due to indefinitely reinvestment 133,020 $ 101,725    
Fiscal Year 2026 [Member]        
Income Taxes [Line Items]        
Expected decrease in unrecognized tax benefits within next 12 months due to lapse of statutes of limitations $ 17,045      
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 $ 55,854 $ 37,249