Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	ADDITIONAL PAID-IN CAPITAL	ACCUMULATED OTHER COMPREHENSIVE LOSS	ACCUMULATED DEFICIT	CONVERTIBLE PREFERRED STOCK	COMMON STOCK
Beginning balance, shares at Dec. 31, 2012					12,120,531	964,880
Beginning balance, value at Dec. 31, 2012	$ (63,919)	$ 0	$ 0	$ (63,920)	$ 60,898	$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options, shares						3,195
Exercise of stock options, value	1	1
Stock-based compensation	2,858	2,858
Accretion from conversion of note payable to related parties	(240)	(240)			240
Accretion of stock issuance costs	(210)	(210)			$ 210
Vesting of restricted stock, shares						124,966
Vesting of restricted stock	0
Conversion of convertible preferred stock to common stock, shares					(12,120,531)	12,120,531
Conversion of convertible preferred stock to common stock	61,348	61,336			$ (61,348)	$ 12
Conversion of convertible notes payable and accrued interest to common stock, shares						1,860,363
Conversion of convertible notes payable and accrued interest to common stock, value	18,604	18,602				$ 2
Issuance of common stock, shares						7,728,000
Issuance of common stock, value	68,226	68,218				$ 8
Issuance of common stock upon net exercise of warrants, shares						483,614
Issuance of common stock upon net exercise of warrants, value	4,896	4,896
Reclassification of warrants from liabilities to equity	6,560	6,560
Unrealized loss on available-for-sale investments	0
Net loss	(31,148)			(31,148)
Ending balance, shares at Dec. 31, 2013					0	23,285,549
Ending balance, value at Dec. 31, 2013	66,976	162,021	0	(95,068)	$ 0	$ 23
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options, shares						579,083
Exercise of stock options, value	9	8				$ 1
Stock-based compensation	9,178	9,178
Issuance of common stock upon exercise of stock purchase rights, shares						10,941
Issuance of common stock upon exercise of stock purchase rights, value	119	119
Accretion from conversion of note payable to related parties	0
Accretion of stock issuance costs	0
Vesting of restricted stock, shares						143,004
Vesting of restricted stock	0
Conversion of convertible notes payable and accrued interest to common stock, value	0
Reclassification of warrants from liabilities to equity	0
Unrealized loss on available-for-sale investments	(107)		(107)
Net loss	(48,133)			(48,133)
Ending balance, shares at Dec. 31, 2014					0	24,018,577
Ending balance, value at Dec. 31, 2014	$ 28,042	171,326	(107)	(143,201)	$ 0	$ 24
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options, shares	297,763					296,716
Exercise of stock options, value	$ 1,282	1,282
Stock-based compensation	12,945	12,945
Issuance of common stock upon exercise of stock purchase rights, shares						5,029
Issuance of common stock upon exercise of stock purchase rights, value	96	96
Accretion from conversion of note payable to related parties	0
Accretion of stock issuance costs	0
Vesting of restricted stock, shares						69,249
Vesting of restricted stock	0
Conversion of convertible notes payable and accrued interest to common stock, value	0
Issuance of common stock, shares						1,754,556
Issuance of common stock, value	50,373	50,371				$ 2
Issuance of common stock upon net exercise of warrants, shares						314,368
Issuance of common stock upon net exercise of warrants, value	9	9
Reclassification of warrants from liabilities to equity	0
Excess tax benefit	463	463
Unrealized loss on available-for-sale investments	(72)		(72)
Net loss	(74,363)			(74,363)
Ending balance, shares at Dec. 31, 2015					0	26,458,495
Ending balance, value at Dec. 31, 2015	$ 18,775	$ 236,492	$ (179)	$ (217,564)	$ 0	$ 26