Fair value measurements - Narrative (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
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	3,300,000		3,300,000		$ 3,600,000
Interest receivable, write-offs	0	0	0	0
Investment at fair value	3,802,000		3,802,000		$ 12,870,000
Other (Expense) Income, Net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) on equity securities	$ (5,800,000)	$ (3,600,000)	$ (9,100,000)	$ (9,800,000)