Subsequent Events - Additional Information (Detail) - USD ($) $ in Thousands			1 Months Ended	12 Months Ended
	Feb. 26, 2019	Jan. 28, 2019	Apr. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Subsequent Event [Line Items]
Net proceeds from sale of ordinary shares				$ 60,000	$ 93,509	$ 30,000
Follow-On Underwritten Public Offering [Member]
Subsequent Event [Line Items]
Shares issued under equity agreement			4,166,667
Gross proceeds from sale of ordinary shares			$ 100,000
Net proceeds from sale of ordinary shares			$ 93,500
Follow-On Underwritten Public Offering [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Shares issued under equity agreement	592,500	3,950,000
Gross proceeds from sale of ordinary shares	$ 22,500	$ 150,100
Net proceeds from sale of ordinary shares		$ 161,600