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Fair value measurements - Additional Information (Detail) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 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  
Reclassification out of accumulated other comprehensive loss 0 $ 0  
Unrealized loss on securities, less than twelve months 189,200,000   $ 787,500,000
Unrealized loss on securities, more than twelve months 411,600,000   315,300,000
Unrealized loss on securities for less than twelve months 100,000   900,000
Unrealized loss on securities for more than twelve months 1,300,000   2,100,000
Investments with other-than-temporary impairment 0   0
Investment at fair value $ 19,083,000   $ 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 14.30%    
Maximum [Member]      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Milestone achievement period 2028    
Milestone discount rates 15.20%    
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