Fair value measurements - Narrative (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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
Interest receivable	$ 1,700,000		1,700,000		$ 3,600,000
Interest receivable, write-offs	0	$ 0	0	0
Investment at fair value	9,527,000		9,527,000		$ 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	$ 1,200,000	$ (3,100,000)	$ (3,300,000)	$ (6,200,000)