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Summary of SARs Activity of the Incentive Plan (Detail) - SARs - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares      
Number of Shares, beginning balance 337,000 351,000 72,000
Granted     300,000
Expired   (2,000)  
Exercised (84,000) (8,000) (2,000)
Forfeited (3,000) (4,000) (19,000)
Number of Shares, ending balance 250,000 337,000 351,000
SARs exercisable 160,000 128,998 22,000
Weighted Average Grant Date FairValue Per Share      
Weighted average fair value, Balance at beginning of period $ 9.15 $ 9.13 $ 8.20
Granted     9.35
Expired   9  
Exercised 8.78 8.20 8.20
Forfeited 9.35 9.35 9.11
Weighted average fair value, Balance at end of period $ 9.28 $ 9.15 $ 9.13