Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022
Assets:
Marketable securities:	$ 90,009	$ 146,847
Restricted cash and cash equivalents	112,297	76,683	$ 97,668	$ 72,290
U.S. government agency securities and treasuries
Assets:
Marketable securities:	63,331	101,625
Commercial paper
Assets:
Marketable securities:	26,678	45,222
Fair Value, Measurements, Recurring
Assets:
Cash and cash equivalents	111,864	74,958
Restricted cash and cash equivalents	433	1,725
Restricted investments	34,168	32,774
Total assets	236,474	256,304
Liabilities:
Contingent consideration		2,415
Total liabilities		2,415
Fair Value, Measurements, Recurring | U.S. government agency securities and treasuries
Assets:
Marketable securities:	63,331	101,625
Fair Value, Measurements, Recurring | Commercial paper
Assets:
Marketable securities:	26,678	45,222
Fair Value, Measurements, Recurring | Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	111,864	74,958
Restricted cash and cash equivalents	433	1,725
Restricted investments	0	0
Total assets	112,297	76,683
Liabilities:
Contingent consideration		0
Total liabilities		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) | Commercial paper
Assets:
Marketable securities:	0	0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Restricted cash and cash equivalents	0	0
Restricted investments	34,168	32,774
Total assets	124,177	179,621
Liabilities:
Contingent consideration		0
Total liabilities		0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | U.S. government agency securities and treasuries
Assets:
Marketable securities:	63,331	101,625
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Commercial paper
Assets:
Marketable securities:	26,678	45,222
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Restricted cash and cash equivalents	0
Restricted investments	0	0
Total assets	0	0
Liabilities:
Contingent consideration		2,415
Total liabilities		2,415
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) | Commercial paper
Assets:
Marketable securities:	$ 0	$ 0