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Recent Accounting Pronouncements - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
New Accounting Pronouncements Or Change in Accounting Principle [Line Items]          
Deferred income taxes   $ (17,891) $ 20,515 $ 9,513  
Income tax (benefit) expense   (17,426) 20,874 $ 9,926  
Correction of prior period adjustment to increase retained earnings         $ 2,300
Restricted cash   $ 3,823 $ 3,559    
Accounting Standards Update 2016-09 [Member]          
New Accounting Pronouncements Or Change in Accounting Principle [Line Items]          
Deferred income taxes $ 400        
Income tax (benefit) expense 300        
Correction of prior period adjustment to increase retained earnings $ 700