Investments - Summarized Fair Value and Gross Unrealized Losses on Securities Available for Sale (Detail) $ in Thousands	Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Number of issues \| Security	311	323
Less than 12 months, Fair value	$ 389,066	$ 430,540
Less than 12 months, Unrealized losses	$ (6,314)	$ (8,415)
12 months or longer, Number of issues \| Security	116	15
12 months or longer, Fair value	$ 181,695	$ 28,018
12 months or longer, Unrealized losses	$ (5,697)	(3,195)
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
12 months or longer, Fair value		$ 9,200
U.S. Government Obligations and Agencies [Member] \| Fixed Maturities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Number of issues \| Security	7	11
Less than 12 months, Fair value	$ 35,464	$ 70,453
Less than 12 months, Unrealized losses	$ (301)	$ (608)
12 months or longer, Number of issues \| Security	9	2
12 months or longer, Fair value	$ 24,140	$ 3,504
12 months or longer, Unrealized losses	$ (576)	$ (62)
Corporate Bonds [Member] \| Fixed Maturities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Number of issues \| Security	159	116
Less than 12 months, Fair value	$ 142,208	$ 96,379
Less than 12 months, Unrealized losses	$ (792)	$ (1,219)
12 months or longer, Number of issues \| Security	39	4
12 months or longer, Fair value	$ 29,796	$ 3,250
12 months or longer, Unrealized losses	$ (516)	$ (80)
Mortgage-Backed and Asset-Backed Securities [Member] \| Fixed Maturities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Number of issues \| Security	83	73
Less than 12 months, Fair value	$ 137,481	$ 149,928
Less than 12 months, Unrealized losses	$ (955)	$ (1,923)
12 months or longer, Number of issues \| Security	37	5
12 months or longer, Fair value	$ 70,218	$ 9,660
12 months or longer, Unrealized losses	$ (1,568)	$ (115)
Municipal Bonds [Member] \| Fixed Maturities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Number of issues \| Security	36	69
Less than 12 months, Fair value	$ 28,265	$ 79,402
Less than 12 months, Unrealized losses	$ (246)	$ (3,726)
12 months or longer, Number of issues \| Security	30
12 months or longer, Fair value	$ 48,370
12 months or longer, Unrealized losses	$ (941)
Redeemable Preferred Stock [Member] \| Fixed Maturities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Number of issues \| Security	21	50
Less than 12 months, Fair value	$ 2,464	$ 6,340
Less than 12 months, Unrealized losses	$ (65)	$ (158)
Common Stock [Member] \| Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Number of issues \| Security	2	1
Less than 12 months, Fair value	$ 17,846	$ 18
Less than 12 months, Unrealized losses	$ (3,820)	$ (7)
12 months or longer, Number of issues \| Security		2
12 months or longer, Fair value		$ 75
12 months or longer, Unrealized losses		$ (115)
Mutual Funds [Member] \| Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Number of issues \| Security	3	3
Less than 12 months, Fair value	$ 25,338	$ 28,020
Less than 12 months, Unrealized losses	$ (135)	$ (774)
12 months or longer, Number of issues \| Security	1	2
12 months or longer, Fair value	$ 9,171	$ 11,529
12 months or longer, Unrealized losses	$ (2,096)	$ (2,823)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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