Marketable securities - Summary of Available for Sale Securities Held (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	$ 1,051,617	$ 859,375
Unrealized gains	1	1
Unrealized losses	(4,705)	(3,579)
Fair value	1,046,913	855,797
U.S. Government Agency Securities and Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	1,042,297	841,895
Unrealized gains	1
Unrealized losses	(4,705)	(3,579)
Fair value	1,037,593	838,316
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	9,320	17,480
Unrealized gains		1
Fair value	$ 9,320	$ 17,481

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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