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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
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward          
Beginning Balance     0 0 0
Granted 0 0 25,771 40,517 40,517
Vested     (25,771)   (40,517)
Forfeited     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
Vested     38.42   29.77
Forfeited     0   0
Non-vested at end of period $ 0   $ 0   $ 0