Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Marketable Securities, Fair value	$ 1,509,028	$ 1,488,848
Equity Securities, Fair value	19,083	22,167
Liabilities:
Contingent consideration	5,526	5,230
Fair Value, Measurements, Recurring [Member]
Assets:
Cash and cash equivalents	221,738	402,579
Equity Securities, Fair value	19,083	22,167
Total assets	1,730,766	1,891,427
Liabilities:
Contingent consideration	5,526	5,230
Total liabilities	5,526	5,230
Fair Value, Measurements, Recurring [Member] \| U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable Securities, Fair value	1,357,845	1,457,601
Fair Value, Measurements, Recurring [Member] \| Certificates of Deposit [Member]
Assets:
Marketable Securities, Fair value	5,960	9,080
Fair Value, Measurements, Recurring [Member] \| Commercial Paper [Member]
Assets:
Marketable Securities, Fair value	42,542
Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member]
Assets:
Marketable Securities, Fair value	83,598
Fair Value, Measurements, Recurring [Member] \| Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	214,346	348,638
Equity Securities, Fair value	19,083	22,167
Total assets	233,429	370,805
Fair Value, Measurements, Recurring [Member] \| Significant other observable inputs (Level 2) [Member]
Assets:
Cash and cash equivalents	7,392	53,941
Total assets	1,497,337	1,520,622
Fair Value, Measurements, Recurring [Member] \| Significant other observable inputs (Level 2) [Member] \| U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable Securities, Fair value	1,357,845	1,457,601
Fair Value, Measurements, Recurring [Member] \| Significant other observable inputs (Level 2) [Member] \| Certificates of Deposit [Member]
Assets:
Marketable Securities, Fair value	5,960	9,080
Fair Value, Measurements, Recurring [Member] \| Significant other observable inputs (Level 2) [Member] \| Commercial Paper [Member]
Assets:
Marketable Securities, Fair value	42,542
Fair Value, Measurements, Recurring [Member] \| Significant other observable inputs (Level 2) [Member] \| Corporate Bonds [Member]
Assets:
Marketable Securities, Fair value	83,598
Fair Value, Measurements, Recurring [Member] \| Significant unobservable inputs (Level 3) [Member]
Liabilities:
Contingent consideration	5,526	5,230
Total liabilities	$ 5,526	$ 5,230

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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Amount of liability recognized arising from contingent consideration in a business combination, expected to be settled beyond one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 25
-Paragraph 6
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
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Fair value of financial and nonfinancial obligations.

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Amount of investment in marketable security.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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