Investments - Cost or Amortized Cost and Fair Value of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	$ 721,755	$ 650,575
Gross Unrealized Gains	1,805	1,201
Gross Unrealized Losses	(12,011)	(11,610)
Fair Value	711,549	640,166
Short-term Investments [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	10,000	5,000
Gross Unrealized Gains		2
Fair Value	10,000	5,002
U.S. Government Obligations and Agencies [Member] \| Fixed Maturities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	60,481	74,937
Gross Unrealized Losses	(877)	(670)
Fair Value	59,604	74,267
Corporate Bonds [Member] \| Fixed Maturities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	228,336	192,328
Gross Unrealized Gains	476	402
Gross Unrealized Losses	(1,308)	(1,300)
Fair Value	227,504	191,430
Mortgage-Backed and Asset-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	221,956	216,679
Fair Value	219,452	214,776
Mortgage-Backed and Asset-Backed Securities [Member] \| Fixed Maturities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	221,956	216,679
Gross Unrealized Gains	19	135
Gross Unrealized Losses	(2,523)	(2,038)
Fair Value	219,452	214,776
Municipal Bonds [Member] \| Fixed Maturities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	120,883	94,794
Gross Unrealized Gains	599	130
Gross Unrealized Losses	(1,187)	(3,727)
Fair Value	120,295	91,197
Redeemable Preferred Stock [Member] \| Fixed Maturities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	12,059	12,723
Gross Unrealized Gains	485	125
Gross Unrealized Losses	(65)	(157)
Fair Value	12,479	12,691
Common Stock [Member] \| Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	22,584	214
Gross Unrealized Gains	47
Gross Unrealized Losses	(3,820)	(121)
Fair Value	18,811	93
Mutual Funds [Member] \| Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	45,456	53,900
Gross Unrealized Gains	179	407
Gross Unrealized Losses	(2,231)	(3,597)
Fair Value	$ 43,404	$ 50,710

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

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
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
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