Financial instruments - Fair Value of Significant Unobservable Inputs (Detail) (Significant Un-Observable Inputs (Level 3) [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	$ 72
Change in fair value of derivative liabilities	7
Ending balance	79
Common stock warrants [Member] | Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	72
Change in fair value of derivative liabilities	7
Ending balance	$ 79