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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Income Taxes [Line Items]      
Operating loss carryforwards, valuation allowance $ 1.9 $ 2.6  
Unrecognized tax benefits if recognized would result in adjustments to primarily deferred taxes $ 4.7 $ 7.5 $ 7.5
Federal corporate tax rate 21.00%    
Blended corporate tax rate 24.50%    
Discrete net tax benefit in connection with initial analysis of tax act $ 22.0    
Decrease in net deferred tax liabilities excluding valuation allowance 22.0    
Estimated increase to valuation allowance 0.5    
Adjustment to deferred income tax benefit 21.5    
AMT credits 2.5    
Accounting Standards Update 2016-09 [Member]      
Income Taxes [Line Items]      
Nondeductible stock compensation expenses 0.4    
Domestic Tax Authority [Member] | 2027-2037 [Member]      
Income Taxes [Line Items]      
Net operating loss carryforwards 415.1    
State and Local Jurisdiction [Member] | 2019-2038 [Member]      
Income Taxes [Line Items]      
Net operating loss carryforwards $ 199.6    
Minimum [Member] | Domestic Tax Authority [Member]      
Income Taxes [Line Items]      
Operating loss carryforwards, expiration year 2027    
Minimum [Member] | State and Local Jurisdiction [Member]      
Income Taxes [Line Items]      
Operating loss carryforwards, expiration year 2019    
Maximum [Member] | Domestic Tax Authority [Member]      
Income Taxes [Line Items]      
Operating loss carryforwards, expiration year 2037    
Maximum [Member] | State and Local Jurisdiction [Member]      
Income Taxes [Line Items]      
Operating loss carryforwards, expiration year 2038