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Summary of Performance option activity (Detail) - $ / shares
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Incentive Stock Options Plans, Additional vesting based on performance 19,760   3,900
Performance Based Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Incentive Stock Option Plans, Beginning balance 81,666    
Incentive Stock Options Plans, Additional vesting based on performance 77,598 77,598  
Incentive Stock Options Plans, Options exercised (17,839)    
Incentive Stock Option Plans, Ending balance 141,425    
Weighted Average Price Per Share, Beginning balance $ 11.49    
Weighted Average Price Per Share, Additional vesting based on performance 11.49    
Weighted Average Price Per Share, Options exercised 11.49    
Weighted Average Price Per Share, Ending balance 11.49    
Exercisable Weighted Average Price Per Share, Beginning balance 11.49    
Exercisable Weighted Average Price Per Share, Ending balance $ 11.49