Subsequent events - Additional Information (Detail) - Celgene Corporation [Member] - USD ($) $ in Millions	Mar. 31, 2016	Feb. 16, 2016	Dec. 31, 2015	Jun. 03, 2015
Option Fee [Member] | Collaborative Arrangement [Member]
Subsequent Event [Line Items]
Amount per product eligible to be received upon achievement of specified event			$ 10.0
Option Fee [Member] | Collaborative Arrangement [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount per product eligible to be received upon achievement of specified event	$ 10.0
Co-Develop and Co-Promote Options not Exercise [Member]
Subsequent Event [Line Items]
Amount per product eligible to be received upon achievement of specified event				$ 10.0
Co-Develop and Co-Promote Options not Exercise [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Amount per product eligible to be received upon achievement of specified event		$ 10.0