Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 130,018	$ 133,724
Unrealized Gains	2	700
Unrealized Losses	(282)	(298)
Fair Value	129,738	134,126
Common Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		300
Unrealized Gains		618
Fair Value		918
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	130,018	133,424
Unrealized Gains	2	82
Unrealized Losses	(282)	(298)
Fair Value	$ 129,738	$ 133,208

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
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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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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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-Publisher FASB
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