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 ($)	Sep. 30, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	$ 401,063,000	$ 425,698,000
Available-for-sale, Gross Unrealized Gains	25,232,000	15,613,000
Available-for-sale, Gross Unrealized Losses	(58,000)	(165,000)
Available-for-sale, Fair Value	426,237,000	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	257,169,000	225,895,000
Available-for-sale, Gross Unrealized Gains	18,235,000	11,906,000
Available-for-sale, Gross Unrealized Losses	(33,000)	(26,000)
Available-for-sale, Fair Value	275,371,000	237,775,000
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	91,162,000	130,453,000
Available-for-sale, Gross Unrealized Gains	5,492,000	3,326,000
Available-for-sale, Gross Unrealized Losses	(25,000)	(1,000)
Available-for-sale, Fair Value	96,629,000	133,778,000
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	23,850,000	29,499,000
Available-for-sale, Gross Unrealized Gains	460,000	64,000
Available-for-sale, Gross Unrealized Losses		(96,000)
Available-for-sale, Fair Value	24,310,000	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	28,882,000	39,851,000
Available-for-sale, Gross Unrealized Gains	1,045,000	317,000
Available-for-sale, Gross Unrealized Losses		(42,000)
Available-for-sale, Fair Value	$ 29,927,000	$ 40,126,000

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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