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Note Stock-based compensation - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized $ 8,900    
Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Period For Recognition 2 years 4 months    
Minimum      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Sharebased Compensation Arrangement By Sharebased Payment Award Options Vested And Expected To Vest Range Based On Shareholder Return 50.00%    
Maximum      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Sharebased Compensation Arrangement By Sharebased Payment Award Options Vested And Expected To Vest Range Based On Shareholder Return 150.00%    
Restricted Stock Units (RSUs)      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award 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      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period 84,616 64,479  
Restricted Stock Expense $ 2,700 $ 2,000  
Employee Service Share Based Compensation Tax Benefit From Compensation Expense 300 $ 200  
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value 4,400    
Windfall Net Of Shortfall 400    
Restricted Stock Units (RSUs) | Executive Officers | Share Vesting On Grant Date      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value $ 3,500    
Restricted Stock Units (RSUs) | Directors      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period 0 0 25,771
Restricted Stock Expense $ 300 $ 300  
Employee Service Share Based Compensation Tax Benefit From Compensation Expense 39 $ 31  
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value $ 0    
Performance Based Shares | Executive Officers      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period 72,414 73,684  
Restricted Stock Expense $ 2,600 $ 1,700  
Employee Service Share Based Compensation Tax Benefit From Compensation Expense $ 300 $ 100  
Share-based Compensation Arrangement by Share-based Payment Award, Award 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. The vesting 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.