Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Marketable securities:	$ 1,113,706	$ 910,752
Equity securities	3,802	12,870
Fair Value, Measurements, Recurring
Assets:
Cash and cash equivalents	324,164	327,214
Equity securities	3,802	12,870
Total	1,437,870	1,237,966
Liabilities:
Contingent consideration	2,386	7,977
Total	2,386	7,977
Fair Value, Measurements, Recurring | U.S. government agency securities and treasuries
Assets:
Marketable securities:	779,058	635,936
Fair Value, Measurements, Recurring | Certificates of deposit
Assets:
Marketable securities:		960
Fair Value, Measurements, Recurring | Corporate bonds
Assets:
Marketable securities:	238,905	186,608
Fair Value, Measurements, Recurring | Commercial paper
Assets:
Marketable securities:	91,941	74,378
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	261,964	311,245
Equity securities	3,802	12,870
Total	265,766	324,115
Liabilities:
Contingent consideration	0	0
Total	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1) | U.S. government agency securities and treasuries
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1) | Certificates of deposit
Assets:
Marketable securities:		0
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1) | Corporate bonds
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1) | Commercial paper
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets:
Cash and cash equivalents	62,200	15,969
Equity securities	0	0
Total	1,172,104	913,851
Liabilities:
Contingent consideration	0	0
Total	0	0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | U.S. government agency securities and treasuries
Assets:
Marketable securities:	779,058	635,936
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Certificates of deposit
Assets:
Marketable securities:		960
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Corporate bonds
Assets:
Marketable securities:	238,905	186,608
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Commercial paper
Assets:
Marketable securities:	91,941	74,378
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Equity securities	0	0
Total	0	0
Liabilities:
Contingent consideration	2,386	7,977
Total	2,386	7,977
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | U.S. government agency securities and treasuries
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Certificates of deposit
Assets:
Marketable securities:		0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Corporate bonds
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Commercial paper
Assets:
Marketable securities:	$ 0	$ 0