Amortized Cost Gross Unrealized Holding Gain or Loss and Fair Value of Available for Sale Security by Major Security Type (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 956,630	$ 622,207
Gross Unrealized Holding Gains	452	100
Gross Unrealized Holding Losses	(3,024)	(1,076)
Aggregate Fair Value	954,058	621,231
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,800	63,919
Gross Unrealized Holding Gains	0	1
Gross Unrealized Holding Losses	0	0
Aggregate Fair Value	2,800	63,920
Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	633,072	358,625
Gross Unrealized Holding Gains	329	20
Gross Unrealized Holding Losses	(2,277)	(732)
Aggregate Fair Value	631,124	357,913
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,075	12,733
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Aggregate Fair Value	16,075	12,733
U.S. Government Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	304,635	186,882
Gross Unrealized Holding Gains	37	0
Gross Unrealized Holding Losses	(747)	(344)
Aggregate Fair Value	303,925	186,538
Greek Government-Issued Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48	48
Gross Unrealized Holding Gains	86	79
Gross Unrealized Holding Losses	0	0
Aggregate Fair Value	$ 134	$ 127

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
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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