Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Equity securities	$ 12,870	$ 22,167
Liabilities:
Contingent consideration	7,977	5,230
U.S. government agency securities and treasuries
Assets:
Marketable securities	635,936	1,457,601
Certificates of deposit
Assets:
Marketable securities	960	9,080
Commercial paper
Assets:
Marketable securities	74,378
Corporate bonds
Assets:
Marketable securities	186,608
Fair value, measurements, recurring
Assets:
Cash and cash equivalents	327,214	402,579
Equity securities	12,870	22,167
Total assets	1,237,966	1,891,427
Liabilities:
Contingent consideration	7,977	5,230
Total liabilities	7,977	5,230
Fair value, measurements, recurring | U.S. government agency securities and treasuries
Assets:
Marketable securities	635,936
Fair value, measurements, recurring | U.S. government agency securities
Assets:
Marketable securities		1,457,601
Fair value, measurements, recurring | Certificates of deposit
Assets:
Marketable securities	960
Fair value, measurements, recurring | Commercial paper
Assets:
Marketable securities	74,378
Fair value, measurements, recurring | Corporate bonds
Assets:
Marketable securities	186,608
Fair value, measurements, recurring | Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	311,245	348,638
Equity securities	12,870	22,167
Total assets	324,115	370,805
Liabilities:
Contingent consideration	0	0
Total liabilities	0	0
Fair value, measurements, recurring | Quoted prices in active markets (Level 1) | U.S. government agency securities and treasuries
Assets:
Marketable securities	0
Fair value, measurements, recurring | Quoted prices in active markets (Level 1) | U.S. government agency securities
Assets:
Marketable securities		0
Fair value, measurements, recurring | Quoted prices in active markets (Level 1) | Certificates of deposit
Assets:
Marketable securities	0	0
Fair value, measurements, recurring | Quoted prices in active markets (Level 1) | Commercial paper
Assets:
Marketable securities	0
Fair value, measurements, recurring | Quoted prices in active markets (Level 1) | Corporate bonds
Assets:
Marketable securities	0
Fair value, measurements, recurring | Significant other observable inputs (Level 2)
Assets:
Cash and cash equivalents	15,969	53,941
Equity securities	0	0
Total assets	913,851	1,520,622
Liabilities:
Contingent consideration	0	0
Total liabilities	0	0
Fair value, measurements, recurring | Significant other observable inputs (Level 2) | U.S. government agency securities and treasuries
Assets:
Marketable securities	635,936
Fair value, measurements, recurring | Significant other observable inputs (Level 2) | U.S. government agency securities
Assets:
Marketable securities		1,457,601
Fair value, measurements, recurring | Significant other observable inputs (Level 2) | Certificates of deposit
Assets:
Marketable securities	960	9,080
Fair value, measurements, recurring | Significant other observable inputs (Level 2) | Commercial paper
Assets:
Marketable securities	74,378
Fair value, measurements, recurring | Significant other observable inputs (Level 2) | Corporate bonds
Assets:
Marketable securities	186,608
Fair value, measurements, recurring | Significant unobservable inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Equity securities	0	0
Total assets	0	0
Liabilities:
Contingent consideration	7,977	5,230
Total liabilities	7,977	5,230
Fair value, measurements, recurring | Significant unobservable inputs (Level 3) | U.S. government agency securities and treasuries
Assets:
Marketable securities	0
Fair value, measurements, recurring | Significant unobservable inputs (Level 3) | U.S. government agency securities
Assets:
Marketable securities		0
Fair value, measurements, recurring | Significant unobservable inputs (Level 3) | Certificates of deposit
Assets:
Marketable securities	0	$ 0
Fair value, measurements, recurring | Significant unobservable inputs (Level 3) | Commercial paper
Assets:
Marketable securities	0
Fair value, measurements, recurring | Significant unobservable inputs (Level 3) | Corporate bonds
Assets:
Marketable securities	$ 0