Investments - Components of Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 79,667	$ 305,915
Unrealized Gains	0	2
Unrealized Losses	(364)	(693)
Fair Value	79,303	305,224
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	76,982	303,521
Unrealized Gains	0	2
Unrealized Losses	(364)	(693)
Fair Value	76,618	302,830
Investments [Member] \| Money Market Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	5,601	18,118
Fair Value	5,601	18,118
Investments [Member] \| Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	0	27,906
Unrealized Losses	0	(39)
Fair Value	0	27,867
Investments [Member] \| US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	7,501	13,295
Unrealized Losses	(34)	(32)
Fair Value	7,467	13,263
Investments [Member] \| Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	63,880	244,202
Unrealized Gains	0	2
Unrealized Losses	(330)	(622)
Fair Value	63,550	243,582
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	2,685	2,394
Fair Value	$ 2,685	$ 2,394

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
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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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