Investments - Cost or Amortized Cost and Fair Value of Debt and Short-term Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	$ 653,888	$ 653,715
Gross Unrealized Gains	728	1,579
Gross Unrealized Losses	(10,378)	(5,960)
Fair Value	644,238	649,334
Short-term Investments [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	10,000	10,000
Fair Value	10,000	10,000
U.S. Government Obligations and Agencies [Member] \| Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	60,073	60,481
Gross Unrealized Losses	(1,276)	(877)
Fair Value	58,797	59,604
Corporate Bonds [Member] \| Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	302,764	228,336
Gross Unrealized Gains	299	476
Gross Unrealized Losses	(4,333)	(1,308)
Fair Value	298,730	227,504
Mortgage-Backed and Asset-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	234,053	221,956
Fair Value	229,499	219,452
Mortgage-Backed and Asset-Backed Securities [Member] \| Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	234,053	221,956
Gross Unrealized Gains	39	19
Gross Unrealized Losses	(4,593)	(2,523)
Fair Value	229,499	219,452
Municipal Bonds [Member] \| Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	34,327	120,883
Gross Unrealized Gains	4	599
Gross Unrealized Losses	(47)	(1,187)
Fair Value	34,284	120,295
Redeemable Preferred Stock [Member] \| Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	12,671	12,059
Gross Unrealized Gains	386	485
Gross Unrealized Losses	(129)	(65)
Fair Value	$ 12,928	$ 12,479

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
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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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
-Name Accounting Standards Codification
-Topic 320
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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
-Name Accounting Standards Codification
-Topic 320
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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