Loan facility - Additional Information (Detail) (Silicon Valley Bank [Member], USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended		3 Months Ended
	Mar. 28, 2014	Oct. 30, 2013	Mar. 31, 2015	Oct. 23, 2013
Line of Credit Facility [Line Items]
Term loan facility with Silicon Valley Bank				$ 4,000
Term loan, amount borrowed		2,500
Warrants expire			2023
Term loan repayment	2,610
Term loan, loss on extinguishment of debt	$ 214
Warrants to Purchase Series C Preferred Stock [Member]
Line of Credit Facility [Line Items]
Warrants to purchase per share				$ 10.56
Warrants to purchase shares of stock, shares				11,363
Warrants to Purchase Common Stock [Member]
Line of Credit Facility [Line Items]
Warrants exercisable			11,363