Amortized Cost Gross Unrealized Holding Gain or Loss and Fair Value of Available for Sale Security by Major Security Type (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 858,357	$ 971,228
Gross Unrealized Holding Gains	552	452
Gross Unrealized Holding Losses	(2,252)	(3,029)
Aggregate Fair Value	856,657	968,651
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	615,435	641,670
Gross Unrealized Holding Gains	426	329
Gross Unrealized Holding Losses	(1,411)	(2,282)
Aggregate Fair Value	614,450	639,717
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,976	16,075
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Aggregate Fair Value	8,976	16,075
U.S. Government Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	233,898	310,635
Gross Unrealized Holding Gains	4	37
Gross Unrealized Holding Losses	(840)	(747)
Aggregate Fair Value	233,062	309,925
Greek Government-Issued Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48	48
Gross Unrealized Holding Gains	122	86
Gross Unrealized Holding Losses	(1)	0
Aggregate Fair Value	$ 169	$ 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.

+ References

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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-Section 50
-Paragraph 2
-Subparagraph (c)
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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
-Subparagraph (SX 210.5-02.2)
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