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, 2018	Dec. 31, 2017
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	$ 473,458	$ 469,739
Available-for-Sale, Gross Unrealized Gains	3,735	7,851
Available-for-Sale, Gross Unrealized Losses	(6,504)	(2,714)
Available-for-Sale, Fair Value	470,689	474,876
Total Fixed Maturity [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	462,033	461,236
Available-for-Sale, Gross Unrealized Gains	3,735	7,072
Available-for-Sale, Gross Unrealized Losses	(6,504)	(2,714)
Available-for-Sale, Fair Value	459,264	465,594
States and Political Subdivisions [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	232,778	244,898
Available-for-Sale, Gross Unrealized Gains	3,730	6,819
Available-for-Sale, Gross Unrealized Losses	(2,410)	(577)
Available-for-Sale, Fair Value	234,098	251,140
Corporate Bonds [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	143,712	130,210
Available-for-Sale, Gross Unrealized Gains	5	224
Available-for-Sale, Gross Unrealized Losses	(943)	(212)
Available-for-Sale, Fair Value	142,774	130,222
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	18,260	18,813
Available-for-Sale, Gross Unrealized Losses	(1,275)	(799)
Available-for-Sale, Fair Value	16,985	18,014
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	67,283	67,315
Available-for-Sale, Gross Unrealized Gains		29
Available-for-Sale, Gross Unrealized Losses	(1,876)	(1,126)
Available-for-Sale, Fair Value	65,407	66,218
Equity Securities [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	11,425	8,503
Available-for-Sale, Gross Unrealized Gains		779
Available-for-Sale, Fair Value	$ 11,425	$ 9,282

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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 investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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