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, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 622,207	$ 167,830
Gross Unrealized Holding Gains	100	74
Gross Unrealized Holding Losses	(1,076)	(342)
Aggregate Fair Value	621,231	167,562
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	63,919	72,302
Gross Unrealized Holding Gains	1	1
Aggregate Fair Value	63,920	72,303
Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	358,625	95,478
Gross Unrealized Holding Gains	20
Gross Unrealized Holding Losses	(732)	(342)
Aggregate Fair Value	357,913	95,136
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,733
Aggregate Fair Value	12,733
U.S. Government Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	186,882
Gross Unrealized Holding Losses	(344)
Aggregate Fair Value	186,538
Greek Government-Issued Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48	50
Gross Unrealized Holding Gains	79	73
Aggregate Fair Value	$ 127	$ 123

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=51813488&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=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
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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.

+ References

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.

+ References

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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