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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, 2019
Mar. 31, 2018
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Granted 84,590 84,616  
Performance Based Shares      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Non-vested at beginning of period 382,186 295,340 295,340
Granted 149,986   239,062
Performance Shares Quantity Adjustment 22,973   234,076
Vested (177,913)   (372,271)
Forfeited     (14,021)
Non-vested at beginning of period 377,232   382,186
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Additional Disclosures Abstract      
Non-vested at beginning of period $ 36.41 $ 30.75 $ 30.75
Granted 55.05   45.81
Performance Shares Quantity Adjustment 55.51   33.09
Vested 46.12   35.83
Forfeited     37.35
Non-vested at end of period $ 40.4   $ 36.41