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Share-Based Employee Compensation - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended 9 Months Ended
Aug. 31, 2018
May 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Aug. 07, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Number of shares available for future grant     3,984,898   3,984,898   3,000,000
Stock option expense     $ 0.9 $ 1.1 $ 3.0 $ 3.3  
Aggregate intrinsic value for outstanding options     4.8   4.8    
Aggregate intrinsic value of exercisable options     4.8   4.8    
Total intrinsic value of options exercised         1.9    
Restricted stock or unit expense     3.1 $ 2.6 $ 8.6 $ 7.5  
Stock Awards [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Number of shares available for future grant             1,500,000
Performance Vesting Stock Options [Member] | Long Term Compensation Plan [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Number of shares available for future grant   62,179          
Reduced percentage of shares earned in stock options plan         66.70%    
Stock based compensation plan, description         The performance criteria for the Fiscal 2019 Employee Performance Stock Option Grant are based upon the achievement of certain levels of return on equity (as defined in the option agreements), ranging from 10.0% to 20.0%, for the fiscal year ending March 31, 2019. All stock options will be earned if the return on equity is 20.0% or greater, and the percentage of shares earned will be reduced proportionately to approximately 66.7% if the return on equity is 10.0%. If the Company does not achieve a return on equity of at least 10.0%, all stock options granted will be forfeited.    
Share-based compensation vesting period   4 years          
Share-based compensation award expiration term   10 years          
Performance Vesting Stock Options [Member] | Long Term Compensation Plan [Member] | Minimum [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Percentage of average return on invested capital   10.00%          
Share-based compensation expiration date         Mar. 31, 2019    
Performance Vesting Stock Options [Member] | Long Term Compensation Plan [Member] | Maximum [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Percentage of average return on invested capital   20.00%          
Share-based compensation expiration date         Mar. 31, 2022    
Time Vesting Stock Options [Member] | Long Term Compensation Plan [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Number of shares available for future grant   51,814          
Share-based compensation vesting period   4 years          
Board of Directors Stock Option [Member] | Long Term Compensation Plan [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Number of shares available for future grant 1,741            
Stock Options [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Unrecognized compensation cost, stock options     7.6   $ 7.6    
Weighted-average period of recognition of unrecognized compensation cost         2 years 6 months    
Performance Vesting Restricted Stock [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Number of shares available for future grant   57,756          
Stock based compensation plan, description         The performance criteria for the Fiscal 2019 Employee Restricted Stock Performance Award are based upon the achievement of certain levels of return on equity (as defined in the award agreement), ranging from 10.0% to 20.0%, for the fiscal year ending March 31, 2019. All restricted shares will be earned if the return on equity is 20.0% or greater, and the percentage of shares earned will be reduced proportionately to approximately 66.7% if the return on equity is 10.0%. If the Company does not achieve a return on equity of at least 10.0%, all awards will be forfeited.    
Share-based compensation vesting period   4 years          
Reduced percentage of restricted shares         66.70%    
Performance Vesting Restricted Stock [Member] | Minimum [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Percentage of average return on invested capital   10.00%          
Share-based compensation vesting date         Mar. 31, 2020    
Performance Vesting Restricted Stock [Member] | Maximum [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Percentage of average return on invested capital   20.00%          
Share-based compensation vesting date         Mar. 31, 2022    
Time Vesting Restricted Stock [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Number of shares available for future grant   48,130          
Share-based compensation vesting period   4 years          
Restricted Stock [Member]              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Number of shares available for future grant 15,950            
Share-based compensation vesting period 6 months            
Weighted-average period of recognition of unrecognized compensation cost         2 years 2 months 12 days    
Weighted average grant date fair value, shares granted         $ 105.13    
Unrecognized compensation cost     $ 21.2   $ 21.2