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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 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward          
Beginning Balance     0 0 0
Granted 22,394 25,771 22,394 25,771 25,771
Vested     (22,394)   (25,771)
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     46.9   38.42
Vested     46.9   38.42
Forfeited     0   0
Non-vested at end of period $ 0   $ 0   $ 0