Fair value measurements - Narrative (Detail) - USD ($)	1 Months Ended	3 Months Ended
	Jan. 31, 2021	Mar. 31, 2021	Mar. 31, 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
Interest receivable		2,200,000		$ 3,100,000
Interest receivable, write-offs		0	0
Investment at fair value		2,707,000		$ 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		$ (28,400,000)	$ (4,500,000)