Schedule of Intangible assets (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Finite Lived Intangible Assets [Line Items]
Acquired and in-licensed rights	$ 50,000
Less: accumulated amortization	(1,645)
Total intangible asset, net	$ 48,355
Acquired and in-licensed rights
Finite Lived Intangible Assets [Line Items]
Estimated useful life	13 years