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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
2 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2017
Jul. 31, 2019
Jul. 31, 2018
Jul. 31, 2019
Jul. 31, 2018
Oct. 31, 2018
Income Tax Disclosure [Line Items]            
Income tax expense (benefit)   $ 1.9 $ 3.8 $ 0.0 $ (7.2)  
Pre-tax (income) loss   25.70% 17.20% (0.20%) (25.80%)  
Net discrete tax expense (benefit) related to provision-to-return adjustments   $ 0.1     $ (1.1)  
Net discrete tax expense (benefit) related to new tax legislation     $ (2.1)      
Net discrete tax expense (benefit) related to share-based compensation       $ 0.8    
Net discrete tax benefits         (14.7)  
Tax reform act, benefit related to remeasurement of net deferred tax liabilities         $ 12.5 $ 11.3
Corporate tax rate 35.00%     21.00%    
Change in valuation allowance   0.0   $ 0.0    
Unrecognized tax benefits   2.5   2.5   $ 2.2
Unrecognized tax benefits that would affect the annual effective income tax rate if recognized   $ 0.2   0.2    
Maximum [Member]            
Income Tax Disclosure [Line Items]            
Income tax expense (benefit)       $ (0.1)