Investments - Components of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 292,862	$ 305,915
Unrealized Gains	4	2
Unrealized Losses	(525)	(693)
Fair Value	292,341	305,224
Investment Type [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	290,362	303,521
Unrealized Gains	4	2
Unrealized Losses	(525)	(693)
Fair Value	289,841	302,830
Investment Type [Member] \| Money Market Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	10,597	18,118
Fair Value	10,597	18,118
Investment Type [Member] \| Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	27,935	27,906
Unrealized Losses	(26)	(39)
Fair Value	27,909	27,867
Investment Type [Member] \| US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	15,305	13,295
Unrealized Losses	(20)	(32)
Fair Value	15,285	13,263
Investment Type [Member] \| Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	236,525	244,202
Unrealized Gains	4	2
Unrealized Losses	(479)	(622)
Fair Value	236,050	243,582
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	2,500	2,394
Fair Value	$ 2,500	$ 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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