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Summary of SARs Activity of the Incentive Plan (Detail) - SARs - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Number of Shares      
Number of Shares, beginning balance 9,000 195,000 250,000
Expired (9,000)    
Exercised   (186,000) (55,000)
Number of Shares, ending balance   9,000 195,000
SARs exercisable   9,000 195,000
Weighted Average Exercise Price      
Weighted average fair value, Balance at beginning of period $ 9.35 $ 9.35 $ 9.28
Expired 9.35    
Exercised   9.35 9.02
Weighted average fair value, Balance at end of period $ 9.35 $ 9.35 $ 9.35