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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
2 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Dec. 31, 2017
Apr. 30, 2019
Apr. 30, 2018
Apr. 30, 2019
Apr. 30, 2018
Oct. 31, 2018
Income Tax Disclosure [Abstract]            
Income tax expense (benefit)   $ 2.5 $ 2.9 $ (1.9) $ (11.0)  
Pre-tax (income) loss   30.80% 27.90% 17.80% (185.80%)  
Net discrete tax expense (benefit) related to share-based compensation   $ 0.4   $ 0.8 $ (2.3)  
Net discrete tax expense (benefit) realted to new tax legislation     $ 0.1      
Net discrete tax benefits         (12.6)  
Tax reform act, benefit related to remeasurement of net deferred tax liabilities         $ 10.4 $ 11.3
Corporate tax rate 35.00%     21.00%    
Change in valuation allowance   0.0   $ 0.0    
Unrecognized tax benefits   2.4   2.4   $ 2.2
Unrecognized tax benefits that would affect the annual effective income tax rate if recognized   $ 0.2   $ 0.2