Statements of Convertible Preferred Stock and Stockholders' (Deficit) Equity - USD ($) $ in Thousands	Total	Junior A Convertible Preferred Stock [Member]	Initial Public Offering [Member]	Private Placement [Member]	Follow On Public Offering [Member]	Common Stock [Member]	Common Stock [Member] Initial Public Offering [Member]	Common Stock [Member] Private Placement [Member]	Common Stock [Member] Follow On Public Offering [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Initial Public Offering [Member]	Additional Paid-in Capital [Member] Private Placement [Member]	Additional Paid-in Capital [Member] Follow On Public Offering [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive (Loss) [Member]	Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member] Junior A Convertible Preferred Stock [Member]	Beneficial Conversion Feature on Convertible Promissory Notes [Member]
Balance, Value at Dec. 31, 2012	$ 58,522															$ 58,522		$ 2,050
Balance, Value at Dec. 31, 2012	$ (58,133)					$ 3				$ 1,248				$ (59,384)
Issuance of stock, Value																	$ 7,642
Balance, Shares at Dec. 31, 2012	26,636,118															26,636,118
Balance, Shares at Dec. 31, 2012						3,328,698
Issuance of stock, Shares																	2,105,263
Preferred stock converted to common shares, Value	$ 4,466					$ 1				3,574				891		$ (4,466)
Preferred stock converted to common shares, Shares						959,545										(2,246,743)
Convertible promissory notes converted to Series D preferred stock, Value																$ 3,888
Convertible promissory notes converted to Series D preferred stock, Shares																2,692,291
Beneficial conversion feature on convertible promissory notes																		1,382
Reclassification of beneficial conversion feature	3,432									3,432								(3,432)
Stock-based compensation expense	123									123
Net loss	(3,963)													(3,963)
Balance, Value at Dec. 31, 2013	65,586	$ 7,642														$ 65,586
Balance, Value at Dec. 31, 2013	$ (54,075)					$ 4				8,377				(62,456)
Balance, Shares at Dec. 31, 2013	29,186,929	2,105,263														29,186,929
Balance, Shares at Dec. 31, 2013						4,288,243
Issuance of stock, Value			$ 56,297	$ 100			$ 6				$ 56,291	$ 100
Issuance of stock, Shares							5,750,000	11,442
Preferred stock converted to common shares, Value	$ 65,586					$ 13				65,573						$ (65,586)
Preferred stock converted to common shares, Shares						12,554,171										(29,186,929)
Stock-based compensation expense	1,371									1,371
Shares issued upon exercise of stock options, Value	128									128
Shares issued upon exercise of stock options, Shares						191,800
Shares issued upon cashless exercise of warrants, Value	0					$ 0				0				0	$ 0	$ 0		$ 0
Shares issued upon cashless exercise of warrants, Shares						6,383
Net loss	(17,745)													(17,745)
Balance, Value at Dec. 31, 2014	$ 51,662					$ 23				131,840				(80,201)
Balance, Shares at Dec. 31, 2014	0
Balance, Shares at Dec. 31, 2014						22,802,039
Issuance of stock, Value					$ 75,231				$ 4				$ 75,227
Issuance of stock, Shares									4,327,956
Stock-based compensation expense	$ 2,514									2,514
Shares issued upon exercise of stock options, Value	362									362
Shares issued upon exercise of stock options, Shares						124,868
Net loss	(24,690)													(24,690)
Other comprehensive income (loss)	(35)														(35)
Balance, Value at Dec. 31, 2015	$ 105,044					$ 27				$ 209,943				$ (104,891)	$ (35)
Balance, Shares at Dec. 31, 2015	0
Balance, Shares at Dec. 31, 2015						27,254,863