Financial instruments - Fair Value of Significant Unobservable Inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - Common stock warrants [Member] - Derivative Financial Instruments, Liabilities [Member] - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	$ 66	$ 72
Change in fair value of derivative liabilities	76	70
Ending balance	$ 142	$ 142