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 ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	$ 348,567	$ 387,665
Available-for-sale, Gross Unrealized Gains	21,583	26,619
Available-for-sale, Gross Unrealized Losses	(260)	(5)
Available-for-sale, Fair Value	369,890	414,279
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	218,857	256,492
Available-for-sale, Gross Unrealized Gains	16,676	20,050
Available-for-sale, Gross Unrealized Losses	(93)
Available-for-sale, Fair Value	235,440	276,542
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	89,352	83,646
Available-for-sale, Gross Unrealized Gains	4,036	5,256
Available-for-sale, Gross Unrealized Losses	(51)	(3)
Available-for-sale, Fair Value	93,337	88,899
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	11,685	18,654
Available-for-sale, Gross Unrealized Gains	249	400
Available-for-sale, Gross Unrealized Losses	(13)	(2)
Available-for-sale, Fair Value	11,921	19,052
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	28,673	28,873
Available-for-sale, Gross Unrealized Gains	622	913
Available-for-sale, Gross Unrealized Losses	(103)
Available-for-sale, Fair Value	$ 29,192	$ 29,786

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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