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	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 74	$ 124
Change in fair value of derivative liabilities	(23)	(73)
Ending balance	$ 51	$ 51