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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
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Granted 166,648 138,516 279,890
Performance Based Shares      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Beginning Balance 295,340 383,982 495,731
Granted 239,062 212,200 344,488
Performance Shares Quantity Adjustment 234,076 (232,989) 39,566
Vested (372,271) (67,853) (487,784)
Forfeited (14,021)   (8,019)
Ending Balance 382,186 295,340 383,982
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Additional Disclosures Abstract      
Non-vested at beginning of period $ 30.75 $ 26.35 $ 28.25
Granted 45.81 42.57 25.86
Performance Shares Quantity Adjustment 33.09 29.1 24.37
Vested 35.83 48.54 27.72
Forfeited 37.35   29.13
Non-vested at end of period $ 36.41 $ 30.75 $ 26.35