Fair value measurements - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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
Fair value, less than twelve months	107,072,000		$ 67,217,000
Unrealized loss on securities, more than twelve months	40,614,000		79,618,000
Unrealized losses, less than twelve months	1,110,000		46,000
Unrealized loss on securities for more than twelve months	15,000		45,000
Interest receivable	2,900,000	3,600,000
Interest receivable, write-offs	0	0
Investment at fair value	$ 8,350,000		$ 12,870,000
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Milestone discount rates	11.80%
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Milestone discount rates	13.50%
Other (Expense) Income, Net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized loss on equity securities	$ 4,500,000	$ 3,100,000