Investments - Cost or Amortized Cost and Fair Value of Debt and Short-term Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	$ 668,868	$ 653,715
Gross Unrealized Gains	773	1,579
Gross Unrealized Losses	(12,879)	(5,960)
Fair Value	656,762	649,334
Short-term Investments [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost		10,000
Fair Value		10,000
U.S. Government Obligations and Agencies [Member] \| Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	59,086	60,481
Gross Unrealized Losses	(1,374)	(877)
Fair Value	57,712	59,604
Corporate Bonds [Member] \| Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	339,730	228,336
Gross Unrealized Gains	424	476
Gross Unrealized Losses	(6,190)	(1,308)
Fair Value	333,964	227,504
Mortgage-Backed and Asset-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	242,492	221,956
Fair Value	237,432	219,452
Mortgage-Backed and Asset-Backed Securities [Member] \| Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	242,492	221,956
Gross Unrealized Gains	31	19
Gross Unrealized Losses	(5,091)	(2,523)
Fair Value	237,432	219,452
Municipal Bonds [Member] \| Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	15,798	120,883
Gross Unrealized Gains		599
Gross Unrealized Losses	(81)	(1,187)
Fair Value	15,717	120,295
Redeemable Preferred Stock [Member] \| Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	11,762	12,059
Gross Unrealized Gains	318	485
Gross Unrealized Losses	(143)	(65)
Fair Value	$ 11,937	$ 12,479

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

No definition available.

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- Definition

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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 and equity securities classified as available-for-sale.

+ References

No definition available.

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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

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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