Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands		Total	Common Stock	Additional Paid-in Capital	Other Comprehensive Loss	Accumulated Deficit
Balance at (in shares) at Dec. 31, 2015			19,570,000
Balances at at Dec. 31, 2015		$ 20,215	$ 2	$ 242,693	$ (29)	$ (222,451)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock net of issuance costs (in shares)			1,333,000
Issuance of common stock net of issuance costs		26,073	$ 0	26,073
Issuance of common stock for employee stock purchase plan (in shares)			139,000
Issuance of common stock for employee stock purchase plan		1,489		1,489
Exercise of stock options (in shares)			349,000
Exercise of stock options		2,607		2,607
Exercise of common stock warrant, net (in shares)			133,000
Exercise of common stock warrants, net		0		0
Vesting of restricted stock units (in shares)			5,000
Stock-based compensation		9,038		9,038
Net loss		(47,089)				(47,089)
Other comprehensive (income) loss		(28)			(28)
Balance at (in shares) at Dec. 31, 2016			21,529,000
Balances at at Dec. 31, 2016		12,305	$ 2	281,900	(57)	(269,540)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock net of issuance costs (in shares)			3,450,000
Issuance of common stock net of issuance costs		56,486		56,486
Issuance of common stock for employee stock purchase plan (in shares)			139,000
Issuance of common stock for employee stock purchase plan		1,793		1,793
Issuance of common stock warrants		674		674
Exercise of stock options (in shares)			228,000
Exercise of stock options		1,086		1,086
Exercise of common stock warrant, net (in shares)			29,000
Exercise of common stock warrants, net		0		0
Vesting of restricted stock units (in shares)			46,000
Stock-based compensation		11,369		11,369
Net loss		(43,562)				(43,562)
Other comprehensive (income) loss		(42)			(42)
Balance at (in shares) at Dec. 31, 2017			25,421,000
Balances at at Dec. 31, 2017		40,109	$ 2	353,308	(99)	(313,102)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock net of issuance costs (in shares)			4,600,000
Issuance of common stock net of issuance costs		53,829	$ 1	53,828
Issuance of common stock for employee stock purchase plan (in shares)			257,000
Issuance of common stock for employee stock purchase plan		1,451		1,451
Issuance of common stock warrants		$ 4,593		4,593
Exercise of stock options (in shares)		430,602	431,000
Exercise of stock options		$ 3,507		3,507
Exercise of common stock warrant, net (in shares)			118,000
Exercise of common stock warrants, net		0		0
Vesting of restricted stock units (in shares)			86,000
Stock-based compensation		11,475		11,475
Net loss		(77,400)				(77,400)
Other comprehensive (income) loss		59			59
Balance at (in shares) at Dec. 31, 2018			30,913,000
Balances at at Dec. 31, 2018		36,869	$ 3	$ 428,162	$ (40)	(391,256)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of accounting policy change	[1]	$ (754)				$ (754)

[1] Effective January 1, 2018, we adopted Accounting Standard Update No. 2014-09, Revenue from Contracts with Customers. See Note 2. Significant Accounting Policies and Note 3. Revenue from Contracts with Customers for more information.