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Note Stock-based compensation (Summary of restricted stock activity under the incentive plan for members of the Board of Directors) (Detail) - Restricted Stock Units (RSUs) - Directors - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Beginning Balance 0 0 0
Granted 25,771 40,517 22,119
Vested (25,771) (40,517) (22,119)
Forfeited 0 0 0
Ending Balance 0 0 0
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Additional Disclosures Abstract      
Non-vested at beginning of period $ 0 $ 0 $ 0
Granted 38.42 29.77 32.29
Vested 38.42 29.77 32.29
Forfeited 0 0 0
Non-vested at end of period $ 0 $ 0 $ 0