Fair Value of Financial Measurements - Schedule of Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	$ 622,923	$ 561,011
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	370	371
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	622,553	560,640
U.S. government and agency securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	43,519	30,593
U.S. government and agency securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	370	371
U.S. government and agency securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	43,149	30,222
States, Municipalities and Political Subdivisions [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	96,925	85,670
States, Municipalities and Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	96,925	85,670
Special Revenue [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	290,086	274,321
Special Revenue [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	290,086	274,321
Hybrid Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	100	100
Hybrid Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	100	100
Industrial and Miscellaneous [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	192,293	170,327
Industrial and Miscellaneous [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt securities, available-for-sale	$ 192,293	$ 170,327