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Share-Based Compensation - Additional Information (Detail)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation expense related to the awards to Non employee directors | $ $ 2,101    
Share-based Compensation Arrangement by Share-based Payment Award, Accelerated Vesting, Number [1] 14,107    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Forfeitures and Expirations 2,569    
Shares reserved for issuance 831,183    
Number of Shares, Granted 7,128 7,515  
Number of shares withheld 10,436 10,269 12,031
Payments of employees' tax obligations to taxing authorities 3    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $ $ 1,100    
SICP Awards to Non-employee directors      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value | $ / shares   $ 71.80  
Amortization of expense equally over a service period 1 year    
Unrecognized compensation expense related to the awards to Non employee directors | $ $ 180    
Number of Shares, Granted 792 835  
Weighted average grant-date fair value of awards granted | $ / shares $ 75.70    
SICP Awards to Key Employees      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value | $ / shares $ 47.39 $ 38.88  
Number of Shares, Granted 49,494 52,355  
Weighted average grant-date fair value of awards granted | $ / shares $ 67.76 $ 63.42  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Nonvested | $ $ 10,700 $ 10,400 $ 7,700
Stock and Incentive Compensation Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares reserved for issuance 475,099    
[1] Includes 2,569 shares that were forfeited