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	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Totals, Amortized Cost	$ 464,740	$ 479,772
Totals, Gross Unrealized Gains	8,215	3,758
Totals, Gross Unrealized Losses	(1,701)	(4,800)
Totals, Fair Value	471,254	478,730
States and Political Subdivisions [Member]
Investment [Line Items]
Totals, Amortized Cost	220,749	231,848
Totals, Gross Unrealized Gains	6,710	3,515
Totals, Gross Unrealized Losses	(395)	(2,118)
Totals, Fair Value	227,064	233,245
Corporate Bonds [Member]
Investment [Line Items]
Totals, Amortized Cost	170,375	173,904
Totals, Gross Unrealized Gains	1,465	243
Totals, Gross Unrealized Losses	(217)	(933)
Totals, Fair Value	171,623	173,214
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Totals, Amortized Cost	12,462	12,835
Totals, Gross Unrealized Losses	(180)	(320)
Totals, Fair Value	12,282	12,515
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Totals, Amortized Cost	61,154	61,185
Totals, Gross Unrealized Gains	40
Totals, Gross Unrealized Losses	(909)	(1,429)
Totals, Fair Value	$ 60,285	$ 59,756

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- Definition

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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

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
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

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
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Section 50
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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