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Investments - Gross Unrealized Gains and Losses and Cost or Amortized Cost, Allowance for Credit Losses and Fair Value of Investments Classified as Available-for-Sale (Detail) - USD ($)
Jun. 30, 2020
Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale, Amortized Cost $ 392,117,000 $ 425,698,000
Available-for-sale, Gross Unrealized Gains 24,170,000 15,613,000
Available-for-sale, Gross Unrealized Losses (47,000) (165,000)
Available-for-sale, Fair Value 416,240,000 441,146,000
Available-for-sale, Allowance for Credit Losses 0 0
States and Political Subdivisions [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale, Amortized Cost 227,932,000 225,895,000
Available-for-sale, Gross Unrealized Gains 16,750,000 11,906,000
Available-for-sale, Gross Unrealized Losses   (26,000)
Available-for-sale, Fair Value 244,682,000 237,775,000
Corporate Bonds [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale, Amortized Cost 103,713,000 130,453,000
Available-for-sale, Gross Unrealized Gains 5,711,000 3,326,000
Available-for-sale, Gross Unrealized Losses (47,000) (1,000)
Available-for-sale, Fair Value 109,377,000 133,778,000
U.S. Agency-Based Mortgage-Backed Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale, Amortized Cost 28,581,000 29,499,000
Available-for-sale, Gross Unrealized Gains 575,000 64,000
Available-for-sale, Gross Unrealized Losses   (96,000)
Available-for-sale, Fair Value 29,156,000 29,467,000
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale, Amortized Cost 31,891,000 39,851,000
Available-for-sale, Gross Unrealized Gains 1,134,000 317,000
Available-for-sale, Gross Unrealized Losses   (42,000)
Available-for-sale, Fair Value $ 33,025,000 $ 40,126,000