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	Sep. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	$ 322,863	$ 387,665
Available-for-sale, Gross Unrealized Gains	19,265	26,619
Available-for-sale, Gross Unrealized Losses	(290)	(5)
Available-for-sale, Fair Value	341,838	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	209,354	256,492
Available-for-sale, Gross Unrealized Gains	15,010	20,050
Available-for-sale, Gross Unrealized Losses	(105)
Available-for-sale, Fair Value	224,259	276,542
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	79,336	83,646
Available-for-sale, Gross Unrealized Gains	3,500	5,256
Available-for-sale, Gross Unrealized Losses	(99)	(3)
Available-for-sale, Fair Value	82,737	88,899
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	9,487	18,654
Available-for-sale, Gross Unrealized Gains	237	400
Available-for-sale, Gross Unrealized Losses	(9)	(2)
Available-for-sale, Fair Value	9,715	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	24,686	28,873
Available-for-sale, Gross Unrealized Gains	518	913
Available-for-sale, Gross Unrealized Losses	(77)
Available-for-sale, Fair Value	$ 25,127	$ 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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