STOCKHOLDERS' EQUITY - Initial Public Offering and Conversion of Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Apr. 02, 2014	Apr. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Preferred stock, shares outstanding			0	0
Proceeds from initial public offering, net of underwriting discounts		$ 80,200	$ 0	$ 80,213	$ 0
Underwriters' discounts and commissions		$ 6,000
Reclassification of deferred offering costs to additional paid-in capital	$ 5,400
Series A Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding	5,680,096
Series B Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding	4,929,749
Series C Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding	9,052,344
Series D Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding	5,619,234
Series E Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding	2,946,105
Common Stock
Class of Stock [Line Items]
Share price (USD per share)		$ 10.00
Convertible preferred stock, shares converted	28,832,898
Number of shares called by warrant	107,876
Common Stock | IPO
Class of Stock [Line Items]
Stock issued during the period		8,625,000
Common Stock | Over-Allotment Option
Class of Stock [Line Items]
Stock issued during the period		1,125,000
Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding	28,227,528
Number of shares called by warrant	103,034