Consolidated Statements of Changes in Convertible Preferred Stock and of Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Series E-1 Convertible Preferred Stock	Series E-2	Series E-3	Series E-4	Series E-5	IPO	Common Stock	Common Stock IPO	Additional Paid-In Capital	Additional Paid-In Capital Series E-1 Convertible Preferred Stock	Additional Paid-In Capital Series E-4	Additional Paid-In Capital Series E-5	Additional Paid-In Capital IPO	Other Comprehensive Income (Loss)	Accumulated Deficit	Accumulated Deficit Series E-1 Convertible Preferred Stock	Accumulated Deficit Series E-2	Accumulated Deficit Series E-3
Beginning balance (in shares) at Dec. 31, 2012	1,517,381
Beginning balance at Dec. 31, 2012	$ 95,433
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of convertible preferred stock	$ 11,256	$ (11,256)				$ 36,375											$ 11,256
Issuance of convertible preferred stock (in shares)				607,476	4,748,484	1,810,441
Conversion of convertible preferred stock			$ (39,000)	$ (24,638)
Conversion of convertible debt					$ 66,954
Issuance of convertible preferred stock as a deemed dividend (in shares)						7,911
Issuance of convertible preferred stock as a deemed dividend						$ 177
Expiration of note payable from stockholder, Series E-1 (in shares)		(1,694)
Expiration of note payable from stockholder, Series E-1		$ (63)
Ending balance (in shares) at Dec. 31, 2013	8,689,999
Ending balance at Dec. 31, 2013	$ 123,982
Beginning balance (in shares) at Dec. 31, 2012								204,024
Beginning balance at Dec. 31, 2012	(216,727)							$ 0		$ 1,599					$ 0	$ (218,326)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense related to stock-based compensation	548									548
Issuance of convertible preferred stock	11,256	(11,256)				36,375											$ 11,256
Conversion of convertible preferred stock			$ 39,000	$ 24,638														$ 39,000	$ 24,638
Conversion of convertible notes					$ 32,008							$ 32,008
Issuance of Series E-5 convertible preferred stock as a deemed dividend						(177)							$ (177)
Issuance of common stock warrants						$ 4,272							$ 4,272
Expiration of note payable from stockholder, Series E-1		$ 63									$ 63
Issuance of common stock upon exercise of stock options (in shares)								4,284
Issuance of common stock upon exercise of stock options	11									11
Issuance of common stock upon net exercise of warrant (in shares)								52,481
Issuance of common stock upon exercise of warrants	7									7
Net loss	(52,448)															(52,448)
Unrealized loss on available for sale securities	0
Ending balance (in shares) at Dec. 31, 2013								260,789
Ending balance at Dec. 31, 2013	$ (157,549)							$ 0		38,331					0	(195,880)
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of preferred stock to common stock in connection with initial public offering (in shares)	(8,689,999)
Conversion of preferred stock to common stock in connection with initial public offering	$ (123,982)
Ending balance (in shares) at Dec. 31, 2014	0
Ending balance at Dec. 31, 2014	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense related to stock-based compensation	6,513									6,513
Conversion of convertible preferred stock	123,981							9		123,972
Conversion of convertible notes	26,206							$ 2		26,204
Issuance of common stock warrants	379						$ 1,441			379				$ 1,441
Issuance of common stock upon exercise of stock options (in shares)								239,000
Issuance of common stock upon exercise of stock options	1,422									1,422
Issuance of common stock upon net exercise of warrant (in shares)								10,613
Net loss	(62,917)															(62,917)
Issuance of common stock and compensation expense relating to employee stock purchase plan (in shares)								25,339
Issuance of common stock relating to employee stock purchase plan	349									349
Conversion of preferred stock to common stock in connection with initial public offering (in shares)								8,689,999
Issuance of common stock (in shares)									6,900,000
Issuance of common stock							$ 98,644		$ 7					$ 98,637
Conversion of convertible notes (in shares)								1,637,846
Issuance of common stock upon net exercise of common stock warrants and related extinguishment of warrant liability in connection with initial public offering (in shares)								1,158,443
Issuance of common stock upon net exercise of common stock warrants and related extinguishment of warrant liability in connection with initial public offering	6,490							$ 1		6,489
Issuance of common stock in connection with follow on offering (in shares)								4,600,000
Issuance of common stock in connection with follow on offering	131,335							$ 5		131,330
Issuance of restricted stock awards, net of repurchase (in shares)								251,325
Issuance of common stock for services rendered (in shares)								1,111
Issuance of common stock for services rendered	17									17
Termination of repurchase rights related to vesting of common stock issued pursuant to early exercises	58									58
Unrealized loss on available for sale securities	$ 0
Ending balance (in shares) at Dec. 31, 2014	23,774,465							23,774,465
Ending balance at Dec. 31, 2014	$ 176,369							$ 24		435,142					0	(258,797)
Ending balance (in shares) at Dec. 31, 2015	0
Ending balance at Dec. 31, 2015	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense related to stock-based compensation	12,388									12,388
Issuance of common stock upon exercise of stock options (in shares)								205,735
Issuance of common stock upon exercise of stock options	2,435									2,435
Issuance of common stock upon net exercise of warrant (in shares)								68,993
Net loss	(73,476)															(73,476)
Issuance of common stock and compensation expense relating to employee stock purchase plan (in shares)								15,745
Issuance of common stock relating to employee stock purchase plan	318									318
Issuance of common stock (in shares)								352,544
Issuance of common stock	10,021									10,021
Issuance of common stock in connection with follow on offering (in shares)								3,737,500
Issuance of common stock in connection with follow on offering	126,230							$ 4		126,226
Issuance of restricted stock awards, net of repurchase (in shares)								169,562
Vested restricted stock awards to pay taxes (in shares)								(36,080)
Vested restricted stock awards to pay taxes	(993)									(993)
Unrealized loss on available for sale securities	$ (40)														(40)
Ending balance (in shares) at Dec. 31, 2015	28,288,464							28,288,464
Ending balance at Dec. 31, 2015	$ 253,252							$ 28		$ 585,537					$ (40)	$ (332,273)