Financial instruments - Fair Value of Significant Unobservable Inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - Warrants to Purchase Common Stock [Member] - Derivative Financial Instruments, Liabilities [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 124	$ 72
Change in fair value of derivative liability	(81)	52
Exercise of warrants	$ (43)
Ending balance		$ 124