Consolidated Statements Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2014	$ 53,001	$ 20	$ 156,576	$ (103,595)
Balance (in shares) at Dec. 31, 2014		20,041,645
Stock-based compensation expense	4,168		4,168
Issuance of common stock	33,256	$ 4	33,252
Issuance of common stock (in shares)		3,691,049
Issuance of common stock from exercise of stock options and employee stock purchase plan	1,804		1,804
Issuance of common stock from exercises of stock options and employee stock purchase plan (in shares)		442,687
Net loss	(45,290)			(45,290)
Balance at Dec. 31, 2015	46,939	$ 24	195,800	(148,885)
Balance (in shares) at Dec. 31, 2015		24,175,381
Stock-based compensation expense	4,848		4,848
Offering costs on issuance of common stock from secondary public offering	(215)		(215)
Issuance of common stock	39,723	$ 6	39,717
Issuance of common stock (in shares)		6,055,341
Issuance of common stock from exercise of stock options and employee stock purchase plan	1,018		1,018
Issuance of common stock from exercises of stock options and employee stock purchase plan (in shares)		251,990
Issuance of warrants	1,829		1,829
Net loss	(54,804)			(54,804)
Balance at Dec. 31, 2016	$ 39,338	$ 30	242,997	(203,689)
Balance (in shares) at Dec. 31, 2016	30,482,712	30,482,712
Stock-based compensation expense	$ 4,790		4,790
Issuance of common stock	18,581	$ 5	18,576
Issuance of common stock (in shares)		5,031,250
Issuance of common stock from exercise of stock options and employee stock purchase plan	1,059	$ 1	1,058
Issuance of common stock from exercises of stock options and employee stock purchase plan (in shares)		434,938
Net loss	(62,428)			(62,428)
Balance at Dec. 31, 2017	$ 1,340	$ 36	$ 267,421	$ (266,117)
Balance (in shares) at Dec. 31, 2017	35,948,900	35,948,900