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Subsequent Events -Summary of Effect of Three-for-Two Stock Split on Common Shares Outstanding (Detail) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Subsequent Event [Line Items]    
Common stock, shares, outstanding 5,718,621 5,658,585
Basic and diluted earnings per common share $ 3.11 $ 1.10
Post Stock Split {Member]    
Subsequent Event [Line Items]    
Common stock, shares, outstanding 8,577,932 8,487,878
Basic and diluted earnings per common share $ 2.07 $ 0.74
Dividends per share on common stock, post-split basis $ 0.29 $ 0.38