Consolidated Statements of Changes in Mandatorily Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (USD $) In Thousands, except Share data	Total	Series A Preferred Stock [Member]	Series B Preferred Stock [Member]	Series C Preferred Stock [Member]	Series D Preferred Stock [Member]	Series E Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Other Comprehensive Income [Member]	Accumulated Deficit [Member]
Balances at beginning of period at Dec. 31, 2011	$ (69,451)									$ (69,451)
Beginning Balance at Dec. 31, 2011		14,383	12,793	35,614	18,167
Balances at beginning of period, shares at Dec. 31, 2011							324,529
Balance, Shares at Dec. 31, 2011		557,339	515,836	3,551,060	2,430,054
Issuance of preferred stock, shares						1,063,951
Accretion of preferred stock	(7,533)							(932)		(6,601)
Issuance of preferred stock, value						15,132
Accretion of preferred stock		1,222	1,072	2,978	2,156	105
Conversion of preferred stock into common stock at initial public offering, shares	8,631,427
Exercise of stock options, shares							85,135
Exercise of stock options	183							183
Exercise of common stock warrant, shares							1,562
Exercise of common stock warrant		0	0	0	0	0
Exercise of common stock warrant, value	4							4
Stock-based compensation	745							745
Net loss	(17,708)									(17,708)
Ending Balance at Dec. 31, 2012		15,605	13,865	38,592	20,323	15,237
Balances at end of period at Dec. 31, 2012	(93,760)									(93,760)
Balance, Shares at Dec. 31, 2012		557,339	515,836	3,551,060	2,430,054	1,063,951
Balances at end of period, shares at Dec. 31, 2012							411,226
Accretion of preferred stock	(4,653)							(862)		(3,791)
Accretion of preferred stock		683	577	1,604	1,140	649
Issuance of common stock from initial public offering, shares							5,400,000
Issuance of common stock from initial public offering, value	46,820							46,820
Conversion of preferred stock into common stock at initial public offering, shares	8,631,427	(557,339)	(515,836)	(3,551,060)	(2,430,054)	(1,063,951)	8,631,427
Conversion of preferred stock to common stock at initial public offering, value	108,275	(16,288)	(14,442)	(40,196)	(21,463)	(15,886)	1	108,274
Conversion of warrants from warrants for preferred stock to warrants for common stock	2,514							2,514
Exercise of stock options, shares							177,165
Exercise of stock options	387							387
Stock-based compensation	1,145							1,145
Net loss	(29,281)									(29,281)
Other comprehensive income	22								22
Balances at end of period at Dec. 31, 2013	31,469						1	158,278	22	(126,832)
Balances at end of period, shares at Dec. 31, 2013	14,620,000						14,619,818
Issuance of common stock from initial public offering, shares							3,318,917
Issuance of common stock from initial public offering, value	56,987						1	56,986
Exercise of stock options, shares	164,394						164,394
Exercise of stock options	411							411
Exercise of common stock warrant, shares							27,269
Exercise of common stock warrant		0	0	0	0	0
Exercise of common stock warrant, value	136							136
Issuance of common stock for employee stock purchase plan, Shares							141,386
Issuance of common stock for employee stock purchase plan, Value	987							987
Stock-based compensation	4,926							4,926
Net loss	(50,038)									(50,038)
Other comprehensive income	(65)								(65)
Balances at end of period at Dec. 31, 2014	$ 44,813						$ 2	$ 221,724	$ (43)	$ (176,870)
Balances at end of period, shares at Dec. 31, 2014	18,272,000						18,271,784