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Income Taxes - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Income Taxes [Line Items]        
Net decrease in valuation allowances $ 16,078 $ 27,194    
Tax credits, net capital and operating loss carryforwards 829,260 797,091    
Tax credits, net capital and operating loss carryforwards having expiration date $ 245,843 $ 244,711    
Tax credits, net capital and operating loss carryforwards expiration date Through 2040 for $245,843 in net operating losses Through 2039 for $244,711 in net operating losses    
Gross unrecognized tax benefits $ 168,186 $ 169,322 $ 187,646 $ 193,024
Accrued income taxes payable for interest and penalties relating to unrecognized tax benefits 15,812 31,108    
Interest and penalty releases net recognized in the income statement 15,396      
Tax benefit recognized in the statements of income, net of interest and penalty reversals   4,679    
Foreign tax benefit resulting from tax enactments related to COVID-19 4,964      
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities 29,400 37,672 3,880  
Release of gross unrecognized tax benefits offset by a net benefit included within income tax expense 14,971      
Tax benefit related to the release of withholding and income tax reserves for unremitting earnings 28,795 $ 28,795 $ 28,795  
Maximum [Member]        
Income Taxes [Line Items]        
Expected decrease in unrecognized tax benefits within next 12 months $ 15,446      
GUERNSEY        
Income Taxes [Line Items]        
Statutory Guernsey tax rate 0.00% 0.00% 0.00%  
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities $ 47,582      
Prepaid Expenses and Other Current Assets [Member]        
Income Taxes [Line Items]        
Tax receivable balance $ 58,336 $ 53,143