Investments - Components of Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 303,029	$ 203,362
Unrealized Gains		21
Unrealized Losses	(876)	(268)
Fair Value	302,153	203,115
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	300,735	200,976
Unrealized Gains		21
Unrealized Losses	(876)	(268)
Fair Value	299,859	200,729
Investments [Member] \| Money Market Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	18,181	68,612
Fair Value	18,181	68,612
Investments [Member] \| Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	5,986
Unrealized Losses	(5)
Fair Value	5,981
Investments [Member] \| US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	11,499
Unrealized Losses	(30)
Fair Value	11,469
Investments [Member] \| Municipal Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis		1,500
Unrealized Gains		2
Fair Value		1,502
Investments [Member] \| Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	265,069	130,864
Unrealized Gains		19
Unrealized Losses	(841)	(268)
Fair Value	264,228	130,615
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	2,294	2,386
Fair Value	$ 2,294	$ 2,386

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

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