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Stock Option Plans - Additional information (Detail) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Mar. 15, 2018
Mar. 17, 2017
Mar. 16, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock Option Plans [Line Items]              
Maximum number of shares available       439,100 476,350 479,167  
Compensation expense related to stock option grants       $ 13.7 $ 17.3 $ 14.7  
Total intrinsic value of options exercised       54.2 $ 33.7 $ 19.3  
Total unrecognized compensation cost related to nonvested options       $ 30.8      
Weighted average period, years       4 years      
Long Term Incentive Plan [Member]              
Stock Option Plans [Line Items]              
Maximum number of shares available       3,300,000      
Shares available for grant       14,400,000      
2014 Long Term Incentive Plan [Member]              
Stock Option Plans [Line Items]              
Number of years options expire from date of grant         7 years 7 years 7 years
Officer and Key Employees [Member] | 2014 Long Term Incentive Plan [Member]              
Stock Option Plans [Line Items]              
Shares available for grant   2,576,000 1,650,400        
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%        
Officer and Key Employees [Member] | 2017 Long Term Incentive Plan [Member]              
Stock Option Plans [Line Items]              
Shares available for grant 1,261,000            
Stock options granted, exercise percentage, on the third anniversary date of the grant 34.00%            
Stock options granted, exercise percentage, on the fourth anniversary date of the grant 33.00%            
Stock options granted, exercise percentage, on the fifth anniversary date of the grant 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      
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%      
Minimum [Member] | 2014 Long Term Incentive Plan [Member]              
Stock Option Plans [Line Items]              
Period of service from grant date stock options awarded not subject to forfeiture         2 years 2 years 2 years
Minimum [Member] | Executive Officer [Member] | 2014 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 55 years
Black-Scholes Option Pricing Model [Member]              
Stock Option Plans [Line Items]              
Weighted average fair value per option for all options       $ 9.27 $ 11.42 $ 8.45