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Income Taxes - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Income Taxes [Line Items]            
Corporate tax rate of the company 21.00% 35.00% 0.00% 0.00% 0.00%  
Net increase (decrease) in valuation allowances     $ 18,154 $ (2,297)    
Tax credits, net capital and operating loss carryforwards $ 894,997   894,997 707,761    
Tax credits, net capital and operating loss carryforwards having expiration date 322,816   $ 322,816 $ 180,882    
Tax credits, net capital and operating loss carryforwards expiration date     Through 2038 for $322,816 in net operating losses Through 2036 for $180,882 in net operating losses    
Gross unrecognized tax benefits 187,646   $ 187,646 $ 193,024 $ 196,668 $ 126,706
Accrued income taxes payable for interest and penalties relating to unrecognized tax benefits 30,807   30,807 28,228    
Accrued income taxes payable for interest and penalties relating to unrecognized tax benefits, tax expense (benefit) recognized in the statements of income, net of interest and penalty reversals     2,579 2,910    
Expected decrease in unrecognized tax benefits within next 12 months 23,869   23,869      
Prepaid Expenses and Other Current Assets [Member]            
Income Taxes [Line Items]            
Tax receivable balance $ 55,198   $ 55,198 $ 38,445