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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, 2016
Dec. 31, 2015
Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Beginning Balance 0 0 0
Granted 40,517 22,119 23,135
Vested (40,517) (22,119) (23,135)
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 29.77 32.29 30.43
Vested 29.77 32.29 30.43
Forfeited 0 0 0
Non-vested at end of period $ 0 $ 0 $ 0