Loan facility - Additional Information (Detail) - Silicon Valley Bank [Member] - USD ($)			6 Months Ended
	Mar. 28, 2014	Oct. 30, 2013	Jun. 30, 2015	Oct. 23, 2013
Line of Credit Facility [Line Items]
Term loan facility with Silicon Valley Bank				$ 4,000,000
Term loan, amount borrowed		$ 2,500,000
Warrants expire			2023
Term loan repayment	$ 2,610,000
Term loan, loss on extinguishment of debt	$ 214,000
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