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	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	$ 424,270	$ 479,772
Available-for-sale, Gross Unrealized Gains	16,092	3,758
Available-for-sale, Gross Unrealized Losses	(145)	(4,800)
Available-for-sale, Fair Value	440,217	478,730
States and Political Subdivisions [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	217,944	231,848
Available-for-sale, Gross Unrealized Gains	12,249	3,515
Available-for-sale, Gross Unrealized Losses	(2)	(2,118)
Available-for-sale, Fair Value	230,191	233,245
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	142,011	173,904
Available-for-sale, Gross Unrealized Gains	3,417	243
Available-for-sale, Gross Unrealized Losses	(7)	(933)
Available-for-sale, Fair Value	145,421	173,214
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	24,458	12,835
Available-for-sale, Gross Unrealized Gains	20
Available-for-sale, Gross Unrealized Losses	(55)	(320)
Available-for-sale, Fair Value	24,423	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	39,857	61,185
Available-for-sale, Gross Unrealized Gains	406
Available-for-sale, Gross Unrealized Losses	(81)	(1,429)
Available-for-sale, Fair Value	$ 40,182	$ 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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-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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-Publisher FASB
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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
-Topic 320
-SubTopic 10
-Section 50
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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