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Note Stock-based compensation - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Stock-based compensation      
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management $ 8,900    
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management, period of recognition (in years) 2 years 5 months    
Minimum      
Stock-based compensation      
Vested and expected to vest range based on shareholder return 50.00%    
Maximum      
Stock-based compensation      
Vested and expected to vest range based on shareholder return 150.00%    
Restricted Stock Units (RSUs)      
Stock-based compensation      
Stock-Based Compensation, vesting rights Unless otherwise stated in an agreement, the compensation cost associated with the shares of restricted stock is determined based on a two-prong vesting schedule. The first part is vested ratably over five years commencing at the date of grant and the second part is vested at termination of employment after attainment of 55 years of age and 10 years of service.  The five-year vesting part is accelerated at termination of employment after attaining 55 years of age and 10 years of service. The vesting schedule for restricted shares granted on or after 2014 was modified as follows, the first part is vested ratably over four years commencing at the date of the grant and the second part is vested at termination of employment after attainment of the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service.  The four year vesting part is accelerated at termination of employment after attaining the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service.    
Restricted Stock Units (RSUs) | Executive Officers      
Stock-based compensation      
Stock granted 84,590 84,616  
Restricted Stock Expense $ 3,800 $ 2,700  
Incentive award, tax benefit 400 $ 300  
Fair Market Value of Restricted Stock 9,800    
Windfall Net Of Shortfall 1,800    
Restricted Stock Units (RSUs) | Executive Officers | Share Vesting On Grant Date      
Stock-based compensation      
Fair Market Value of Restricted Stock $ 5,000    
Restricted Stock Units (RSUs) | Directors      
Stock-based compensation      
Stock granted 1,052 0 25,159
Restricted Stock Expense $ 100 $ 300  
Incentive award, tax benefit 6 $ 39  
Fair Market Value of Restricted Stock $ 100    
Performance Based Shares | Executive Officers      
Stock-based compensation      
Stock granted 65,396 72,414  
Restricted Stock Expense $ 3,600 $ 2,600  
Incentive award, tax benefit $ 300 $ 300  
Stock-Based Compensation, vesting rights The number of shares that will ultimately vest ranges from 50% to a 150% of target based on both market (TSR) and performance (EPS) conditions. The performance shares vest at the end of the three-year performance cycle. If a participant terminates employment after attaining the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service, the performance shares shall continue outstanding and vest at the end of the performance cycle.