Fair value measurements - Narrative (Detail) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized gain (loss) on available-for-sale securities		$ 0	$ 0	$ 0	$ 0
Interest receivable		1,900,000		1,900,000		$ 3,100,000
Interest receivable, write-offs		0	0	0	0
Investment at fair value		2,620,000		2,620,000		$ 5,653,000
Proceeds from sale of equity securities	$ 31,300,000
Other (Expense) Income, Net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) on equity securities		$ 100,000	$ 1,200,000	$ 28,300,000	$ (3,300,000)