Investments - Components of Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 219,351	$ 305,915
Unrealized Gains	3	2
Unrealized Losses	(307)	(693)
Fair Value	219,047	305,224
Investment Type [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	216,721	303,521
Unrealized Gains	3	2
Unrealized Losses	(307)	(693)
Fair Value	216,417	302,830
Investment Type [Member] \| Money Market Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	5,278	18,118
Fair Value	5,278	18,118
Investment Type [Member] \| Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	9,423	27,906
Unrealized Losses	(10)	(39)
Fair Value	9,413	27,867
Investment Type [Member] \| US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	13,505	13,295
Unrealized Losses	(14)	(32)
Fair Value	13,491	13,263
Investment Type [Member] \| Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	188,515	244,202
Unrealized Gains	3	2
Unrealized Losses	(283)	(622)
Fair Value	188,235	243,582
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	2,630	2,394
Fair Value	$ 2,630	$ 2,394

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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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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