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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 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward          
Granted 70,690 74,037 155,306 138,516  
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     227,720   212,200
Performance Shares Quantity Adjustment     160,693   (232,989)
Vested     (280,733)   (67,853)
Forfeited     (2,326)    
Ending Balance 400,694   400,694   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     45.48   42.57
Performance Shares Quantity Adjustment     30.24   29.1
Vested     34.48   48.54
Forfeited     33.07    
Non-vested at end of period $ 36.29   $ 36.29   $ 30.75