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Summary of Performance option activity (Detail) - Performance Based Stock Options - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Incentive Stock Option Plans, Beginning balance 81,666 82,334
Incentive Stock Option Plans, Option forfeited   (668)
Incentive Stock Option Plans, Additional vesting based on performance 77,598  
Incentive Stock Option Plans, Option exercised (18,853)  
Incentive Stock Option Plans, Ending balance 140,411 81,666
Weighted Average Price Per Share, Beginning balance $ 11.49 $ 11.49
Weighted Average Price Per Share, Option forfeited   11.49
Weighted Average Price Per Share, Additional vesting based on performance 11.49  
Weighted Average Price Per Share, Option exercised 11.49  
Weighted Average Price Per Share, Ending balance 11.49 11.49
Exercisable Weighted Average Price Per Share, Beginning balance 11.49 0
Exercisable Weighted Average Price Per Share, Additional vesting based on performance 11.49  
Exercisable Weighted Average Price Per Share, Option exercised 11.49  
Exercisable Weighted Average Price Per Share, Ending balance $ 11.49 $ 11.49