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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, 2017
Mar. 31, 2016
Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Granted 64,479 161,500  
Performance Based Shares      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Beginning Balance 383,982 495,731 495,731
Granted 138,163   344,488
Quantity adjusted by TSR factor (55,515)   39,566
Vested (67,983)   (487,784)
Forfeited     (8,019)
Ending Balance 398,647   383,982
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Additional Disclosures Abstract      
Non-vested at beginning of period $ 26.35 $ 28.25 $ 28.25
Granted 42.69   25.86
Quantity adjusted by TSR factor 36.09   24.37
Vested 40.28   27.72
Forfeited     29.13
Non-vested at end of period $ 28.29   $ 26.35