Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Assets:
Marketable securities:	$ 147,733	$ 196,652
Restricted cash and cash equivalents	195,084	72,290	$ 271,814	$ 130,448
U.S. government agency securities and treasuries
Assets:
Marketable securities:	96,167	118,779
Corporate bonds
Assets:
Marketable securities:	987	2,498
Commercial paper
Assets:
Marketable securities:	50,579	75,375
Fair value, measurements, recurring
Assets:
Cash and cash equivalents	193,629	71,032
Restricted cash and cash equivalents	1,455	1,257
Restricted investments	31,941	31,768
Total assets	374,758	300,709
Liabilities:
Contingent consideration	2,253
Total liabilities	2,253	2,180
Fair value, measurements, recurring | U.S. government agency securities and treasuries
Assets:
Marketable securities:	96,167	118,779
Fair value, measurements, recurring | Corporate bonds
Assets:
Marketable securities:	987	2,498
Fair value, measurements, recurring | Commercial paper
Assets:
Marketable securities:	50,579	75,375
Fair value, measurements, recurring | Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	175,713	71,032
Restricted cash and cash equivalents	1,455	1,257
Restricted investments	0	0
Total assets	177,168	72,289
Liabilities:
Contingent consideration	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	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	17,916	0
Restricted cash and cash equivalents	0	0
Restricted investments	31,941	31,768
Total assets	197,590	228,420
Liabilities:
Contingent consideration	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:	96,167	118,779
Fair value, measurements, recurring | Significant other observable inputs (Level 2) | Corporate bonds
Assets:
Marketable securities:	987	2,498
Fair value, measurements, recurring | Significant other observable inputs (Level 2) | Commercial paper
Assets:
Marketable securities:	50,579	75,375
Fair value, measurements, recurring | Significant unobservable inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Restricted cash and cash equivalents	0	0
Restricted investments	0	0
Total assets	0	0
Liabilities:
Contingent consideration	2,253
Total liabilities	2,253	2,180
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