Fair value measurements - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents maturities	90 days or less
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2021
Milestone discount rates	16.20%
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2028
Milestone discount rates	17.00%