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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, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
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 0 1,994 40,517 20,023 22,119
Vested     (40,517)   (22,119)
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     29.77   32.29
Vested     29.77   32.29
Forfeited     0   0
Non-vested at end of period $ 0   $ 0   $ 0