Fair value measurements - Narrative (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Realized gain (loss) recognized on sale		$ 0	$ 0
Interest receivable		300,000	1,500,000
Interest receivable, write-off		0	0
Investment at fair value		3,691,000	5,653,000
Proceeds from sale of equity securities	$ 31,300,000
Unrealized gain (loss) on equity securities		$ 29,400,000	$ (7,200,000)	$ (9,300,000)