Amortized Cost Gross Unrealized Holding Gain or Loss and Fair Value of Available for Sale Security by Major Security Type (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 865,418	$ 956,630
Gross Unrealized Holding Gains	503	452
Gross Unrealized Holding Losses	(2,472)	(3,024)
Aggregate Fair Value	863,449	954,058
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,800
Gross Unrealized Holding Gains		0
Gross Unrealized Holding Losses		0
Aggregate Fair Value		2,800
Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	603,611	633,072
Gross Unrealized Holding Gains	395	329
Gross Unrealized Holding Losses	(1,713)	(2,277)
Aggregate Fair Value	602,293	631,124
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,234	16,075
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Aggregate Fair Value	4,234	16,075
U.S. Government Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	257,525	304,635
Gross Unrealized Holding Gains	15	37
Gross Unrealized Holding Losses	(759)	(747)
Aggregate Fair Value	256,781	303,925
Greek Government-Issued Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48	48
Gross Unrealized Holding Gains	93	86
Gross Unrealized Holding Losses	0	0
Aggregate Fair Value	$ 141	$ 134

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
-Subparagraph (aa)
-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
-SubTopic 10
-Section 45
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-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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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
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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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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+ References

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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