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Stock Option Plans - Additional information (Detail) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 9 Months Ended
Mar. 14, 2019
Mar. 15, 2018
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2019
Sep. 30, 2018
Stock Option Plans [Line Items]                    
Maximum number of shares available         399,900     420,200    
Compensation expense related to stock option grants     $ 3.4 $ 3.5 $ 3.6 $ 3.7 $ 1.6 $ 4.8 $ 10.5 $ 10.1
Total intrinsic value of options exercised                 60.9 $ 47.2
Total unrecognized compensation cost related to nonvested options     $ 32.2           $ 32.2  
Weighted average period, years                 4 years  
Long Term Incentive Plan [Member]                    
Stock Option Plans [Line Items]                    
Maximum number of shares available                 2,800,000  
Shares available for grant     13,200,000           13,200,000  
Number of years options expire from date of grant                 7 years  
Officer and Key Employees [Member] | Long Term Incentive Plan [Member]                    
Stock Option Plans [Line Items]                    
Shares available for grant 1,283,300 1,261,000                
Stock options granted, exercise percentage, on the third anniversary date of the grant 34.00% 34.00%                
Stock options granted, exercise percentage, on the fourth anniversary date of the grant 33.00% 33.00%                
Stock options granted, exercise percentage, on the fifth anniversary date of the grant 33.00% 33.00%                
Maximum [Member] | Long Term Incentive Plan [Member]                    
Stock Option Plans [Line Items]                    
Maximum period for the exercise of stock options, years                 7 years 7 years
Minimum [Member] | Long Term Incentive Plan [Member]                    
Stock Option Plans [Line Items]                    
Minimum exercise price of stock options, percent of fair market value of a share of common stock on the date of grant                 100.00%  
Period of service from grant date stock options awarded not subject to forfeiture                 2 years 2 years
Minimum [Member] | Executive Officer [Member] | Long Term Incentive Plan [Member]                    
Stock Option Plans [Line Items]                    
Minimum age of employee with not subject to award forfeiture on condition compliance                 55 years 55 years
Black-Scholes Option Pricing Model [Member]                    
Stock Option Plans [Line Items]                    
Weighted average fair value per option for all options                 $ 10.71 $ 9.27