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Summary of Performance-based Award Activity (Detail) - Performance Shares
6 Months Ended
Jun. 30, 2017
USD ($)
$ / shares
shares
Number of shares  
Beginning balance | shares 145,962 [1]
Grants | shares 87,000 [1]
Cancellations | shares (792) [1]
Ending balance | shares 232,170 [1]
Total unrecognized compensation expense remaining | $ $ 2,511,000 [1]
Weighted-average years expected to be recognized over 2 years 1 month 6 days [1]
Weighted-average grant date fair value  
Beginning balance | $ / shares $ 15.32
Grants | $ / shares 18.45
Cancellations | $ / shares 15.17
Ending balance | $ / shares $ 16.49
[1] Represents the target number of shares to be issued for each performance-based award.