Fair value measurements - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents maturities			90 days or less
Realized gain (loss) on available-for-sale securities	$ 0	$ 0	$ 0	$ 0
Reclassification out of accumulated other comprehensive loss	0	$ 0	0	$ 0
Unrealized loss on securities, less than twelve months	52,400,000		52,400,000		$ 787,500,000
Unrealized loss on securities, more than twelve months	229,200,000		229,200,000		315,300,000
Unrealized loss on securities for less than twelve months	100,000		100,000		900,000
Unrealized loss on securities for more than twelve months	500,000		500,000		2,100,000
Investments with other-than-temporary impairment	0		0		0
Investment at fair value	15,983,000		$ 15,983,000		$ 22,167,000
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2021
Milestone discount rates			14.20%
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2028
Milestone discount rates			15.00%
Other (Expense) Income, Net [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized loss on equity securities	$ 3,100,000		$ 6,200,000