Fair value measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized loss on debt securities, less than twelve months	$ 67,200,000	$ 787,500,000
Unrealized loss on debt securities, more than twelve months	79,600,000	315,300,000
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 Months, accumulated loss	100,000	900,000
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	100,000	2,100,000
Investments with other-than-temporary impairment	0	0
Investment at fair value	$ 12,870,000	22,167,000
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Milestone discount rates	12.70%
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Milestone discount rates	14.30%
Other Income (Expense), Net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain on equity securities	$ 9,300,000	$ 2,200,000