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, 2017	Dec. 31, 2016
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	$ 471,391	$ 489,871
Available-for-Sale, Gross Unrealized Gains	7,589	8,156
Available-for-Sale, Gross Unrealized Losses	(2,643)	(5,567)
Available-for-Sale, Fair Value	476,337	492,460
Other Investments [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	1,000	10,000
Available-for-Sale, Gross Unrealized Gains	280	3,330
Available-for-Sale, Fair Value	1,280	13,330
Total Fixed Maturity [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	462,892	479,871
Available-for-Sale, Gross Unrealized Gains	7,007	4,793
Available-for-Sale, Gross Unrealized Losses	(2,643)	(5,567)
Available-for-Sale, Fair Value	467,256	479,097
States and Political Subdivisions [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	236,666	231,168
Available-for-Sale, Gross Unrealized Gains	6,229	4,340
Available-for-Sale, Gross Unrealized Losses	(1,243)	(3,215)
Available-for-Sale, Fair Value	241,652	232,293
Corporate Bonds [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	151,339	182,350
Available-for-Sale, Gross Unrealized Gains	699	436
Available-for-Sale, Gross Unrealized Losses	(54)	(271)
Available-for-Sale, Fair Value	151,984	182,515
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	13,408	10,428
Available-for-Sale, Gross Unrealized Gains	8	17
Available-for-Sale, Gross Unrealized Losses	(748)	(1,103)
Available-for-Sale, Fair Value	12,668	9,342
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	61,479	55,925
Available-for-Sale, Gross Unrealized Gains	71
Available-for-Sale, Gross Unrealized Losses	(598)	(978)
Available-for-Sale, Fair Value	60,952	54,947
Equity Securities [Member]
Investment [Line Items]
Available-for-Sale, Cost or Amortized Cost	7,499
Available-for-Sale, Gross Unrealized Gains	302	33
Available-for-Sale, Fair Value	$ 7,801	$ 33

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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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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-Publisher FASB
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-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.

+ References

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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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