Financial instruments - Summary of Short-Term Investments Consist of Available-for-Sale Securities (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Due within one year, Amortized cost		$ 56,123,000
Due within one year, Fair value		56,053,000
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 0
Short-term Investments [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		56,123,000
Gross unrealized gains		0
Gross unrealized losses		(70,000)
Fair value		$ 56,053,000