Investments - Gross Unrealized Gains and Losses and Cost or Amortized Cost and Fair Value of Investments Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	$ 425,698	$ 479,772
Available-for-sale, Gross Unrealized Gains	15,613	3,758
Available-for-sale, Gross Unrealized Losses	(165)	(4,800)
Available-for-sale, Fair Value	441,146	478,730
States and Political Subdivisions [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	225,895	231,848
Available-for-sale, Gross Unrealized Gains	11,906	3,515
Available-for-sale, Gross Unrealized Losses	(26)	(2,118)
Available-for-sale, Fair Value	237,775	233,245
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	130,453	173,904
Available-for-sale, Gross Unrealized Gains	3,326	243
Available-for-sale, Gross Unrealized Losses	(1)	(933)
Available-for-sale, Fair Value	133,778	173,214
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	29,499	12,835
Available-for-sale, Gross Unrealized Gains	64
Available-for-sale, Gross Unrealized Losses	(96)	(320)
Available-for-sale, Fair Value	29,467	12,515
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	39,851	61,185
Available-for-sale, Gross Unrealized Gains	317
Available-for-sale, Gross Unrealized Losses	(42)	(1,429)
Available-for-sale, Fair Value	$ 40,126	$ 59,756

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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-SubTopic 10
-Section 50
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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).

+ References

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Publisher FASB
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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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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