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Income Taxes - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Income Taxes [Line Items]        
Deferred income tax expenses as a result of enacted changes in tax laws or rates $ 4,817      
Corporate tax rate of the company 0.00% 0.00% 0.00%  
Net increase (decrease) in valuation allowances $ (2,297) $ (15,295)    
Tax credits, net capital and operating loss carryforwards 707,761 773,721    
Tax credits, net capital and operating loss carryforwards having expiration date $ 180,882 214,538    
Tax credits, net capital and operating loss carryforwards expiration date Through 2036 for $180,882 in net operating losses      
Gross unrecognized tax benefits $ 193,024 196,668 $ 126,706 $ 123,942
Accrued income taxes payable for interest and penalties relating to unrecognized tax benefits 28,228 31,138    
Accrued income taxes payable relating to unrecognized tax expense (benefit) recognized in the statements of income (2,910) $ 8,919    
Expected decrease in unrecognized tax benefits within next 12 months $ 17,360      
September 30, 2016 [Member]        
Income Taxes [Line Items]        
Tax credits, net capital and operating loss carryforwards expiration date Through 2036 for $214,538 in net operating losses