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Note Stock-based compensation (Summary of restricted stock activity under the incentive plan for members of management) (Detail) - Restricted Stock Units (RSUs) - Executive Officers - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Granted 84,616 64,479  
Performance Based Shares      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Beginning Balance 295,340 383,982 383,982
Granted 157,030   212,200
Performance Shares Quantity Adjustment 106,499   (232,989)
Vested (171,636)   (67,853)
Forfeited (2,326)    
Ending Balance 384,907   295,340
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Additional Disclosures Abstract      
Non-vested at beginning of period $ 30.75 $ 26.35 $ 26.35
Granted 44.7   42.57
Performance Shares Quantity Adjustment 30.37   29.1
Vested 36   48.54
Forfeited 33.07    
Non-vested at end of period $ 33.98   $ 30.75