CONSOLIDATED STATEMENTS OF CONVERTIBLE REDEEMABLE PREFERRED STOCK AND SHAREHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Series A convertible redeemable preferred stock	Series B convertible redeemable preferred stock	Series C convertible redeemable preferred stock	Series D convertible redeemable preferred stock	Common Stock	Additional Paid-In Capital	Treasury Stock, at cost	Accumulated Deficit	Total
Balance at beginning of year at Dec. 31, 2014	$ 12,781	$ 51,212	$ 13,114		$ 1	$ 12,407	$ (3)	$ (101,753)
Balance at beginning of year at Dec. 31, 2014									$ (89,348)
Balance at beginning of year (in shares) at Dec. 31, 2014	9,232,334	27,324,237	8,658,008		1,006,219
Increase (decrease) in convertible redeemable preferred stock
Exercise of common stock options						517			517
Exercise of common stock options, shares					173,251
Exercise of warrants						6			6
Exercise of warrants (in shares)					16,062
Issuance of restricted stock awards to employees, shares					194,694
Accruals of dividends and accretion to redemption value	$ 2,297	$ 18,034	$ 2,996	$ 1,245		(24,572)			(24,572)
Issuance of convertible redeemable preferred stock, net of issuance costs				$ 44,807
Issuance of convertible redeemable preferred stock, net of issuance costs, shares				37,500,000
Conversion of notes to convertible redeemable preferred stock				$ 5,000
Conversion of notes to convertible redeemable preferred stock, shares				4,166,667
Extinguishment of prior preferred stock dividends	(3,733)	(23,341)	(4,110)			31,184			31,184
Conversion of preferred stock to common stock	$ (11,345)	$ (45,905)	$ (12,000)	$ (50,000)	$ 13	119,237			119,250
Conversion of preferred stock to common stock, shares	(9,232,334)	(27,324,237)	(8,658,008)	(41,666,667)	12,591,463
Public offering of common stock, net of issuance costs					$ 7	72,022			72,029
Public offering of common stock, net of issuance costs (in shares)					6,670,000
Issuance of common stock in payment of accrued dividends				$ 1,052		1,052			1,052
Issuance of common stock in payment of accrued dividends, shares					87,662
Stock-based compensation expense						2,209			2,209
Net loss								(27,255)	(27,255)
Balance at end of year at Dec. 31, 2015					$ 21	214,062	(3)	(129,008)
Balance at end of year at Dec. 31, 2015									85,072
Balance at end of year (in shares) at Dec. 31, 2015					20,739,351
Increase (decrease) in convertible redeemable preferred stock
Exercise of common stock options						443			443
Exercise of common stock options, shares					81,831
Issuance for employee stock purchase plan						442			442
Issuance for employee stock purchase plan, shares					42,918
Public offering of common stock, net of issuance costs					$ 8	137,332			137,340
Public offering of common stock, net of issuance costs (in shares)					8,500,000
Retirement of treasury stock						(3)	$ 3
Stock-based compensation expense						5,787			5,787
Net loss								(94,176)	(94,176)
Balance at end of year at Dec. 31, 2016					$ 29	358,063		(223,184)
Balance at end of year at Dec. 31, 2016									134,908
Balance at end of year (in shares) at Dec. 31, 2016					29,364,100
Increase (decrease) in convertible redeemable preferred stock
Exercise of common stock options					$ 1	735			$ 736
Exercise of common stock options, shares					158,801				158,801
Issuance for employee stock purchase plan						1,141			$ 1,141
Issuance for employee stock purchase plan, shares					110,841
Vesting of restricted stock units (in shares)					14,757
Shares withheld for employee taxes upon vesting of restricted stock units						(68)			(68)
Shares withheld for employee taxes upon vesting of restricted stock units (Shares)					(4,819)
Public offering of common stock, net of issuance costs					$ 3	34,280			$ 34,283
Public offering of common stock, net of issuance costs (in shares)					3,126,998				3,126,998
Stock-based compensation expense						7,945			$ 7,945
Net loss								(74,865)	(74,865)
Balance at end of year at Dec. 31, 2017					$ 33	$ 402,096		$ (298,049)
Balance at end of year at Dec. 31, 2017									$ 104,080
Balance at end of year (in shares) at Dec. 31, 2017					32,770,678