Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Assets:
Equity securities	$ 3,691	$ 5,653
Corporate bonds
Assets:
Marketable securities	49,307	98,781
Commercial paper
Assets:
Marketable securities	54,065	38,975
Fair value, measurements, recurring
Assets:
Cash and cash equivalents	161,160	260,629
Equity securities	3,691	5,653
Total assets	396,617	741,673
Fair value, measurements, recurring | U.S. government agency securities and treasuries
Assets:
Marketable securities	128,393	337,635
Fair value, measurements, recurring | Corporate bonds
Assets:
Marketable securities	49,308	98,781
Fair value, measurements, recurring | Commercial paper
Assets:
Marketable securities	54,065	38,975
Fair value, measurements, recurring | Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	161,146	260,629
Equity securities	3,691	5,653
Total assets	164,837	266,282
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) | 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	14	0
Equity securities	0	0
Total assets	231,780	475,391
Fair value, measurements, recurring | Significant other observable inputs (Level 2) | U.S. government agency securities and treasuries
Assets:
Marketable securities	128,393	337,635
Fair value, measurements, recurring | Significant other observable inputs (Level 2) | Corporate bonds
Assets:
Marketable securities	49,308	98,781
Fair value, measurements, recurring | Significant other observable inputs (Level 2) | Commercial paper
Assets:
Marketable securities	54,065	38,975
Fair value, measurements, recurring | Significant unobservable inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Equity securities	0	0
Total assets	0	0
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) | Corporate bonds
Assets:
Marketable securities	0	0
Fair value, measurements, recurring | Significant unobservable inputs (Level 3) | Commercial paper
Assets:
Marketable securities	$ 0	$ 0