Investments - Components of Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 305,915	$ 303,029
Unrealized Gains	2
Unrealized Losses	(693)	(876)
Fair Value	305,224	302,153
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	303,521	300,735
Unrealized Gains	2
Unrealized Losses	(693)	(876)
Fair Value	302,830	299,859
Investments [Member] \| Money Market Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	18,118	18,181
Fair Value	18,118	18,181
Investments [Member] \| Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	27,906	5,986
Unrealized Losses	(39)	(5)
Fair Value	27,867	5,981
Investments [Member] \| US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	13,295	11,499
Unrealized Losses	(32)	(30)
Fair Value	13,263	11,469
Investments [Member] \| Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	244,202	265,069
Unrealized Gains	2
Unrealized Losses	(622)	(841)
Fair Value	243,582	264,228
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	2,394	2,294
Fair Value	$ 2,394	$ 2,294

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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