Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2014	$ 52,174	$ 50	$ 8	$ 60,222	$ (8,106)
Balance, shares at Dec. 31, 2014		10	8,427
Issuance of common stock, net of issuance costs	12,896	$ 0	$ 1	12,895	0
Issuance of common stock, net of issuance costs, shares		0	342
Exercise of stock options	$ 1,514	$ 0	$ 0	1,514	0
Exercise of stock options, shares	111	0	111
Issuance of restricted common stock	$ 0	$ 0	$ 0	0	0
Issuance of restricted common stock, shares		0	150
Forfeiture of restricted common stock	0	$ 0	$ 0	0	0
Forfeiture of restricted common stock, shares		0	(31)
Purchase and retirement of common stock	(2,443)	$ 0	$ 0	(2,443)	0
Purchase and retirement of common stock, shares		0	(80)
Stock-based compensation	5,251	$ 0	$ 0	5,251	0
Net loss	(17,934)	0	0	0	(17,934)
Balance at Dec. 31, 2015	51,458	$ 50	$ 9	77,439	(26,040)
Balance, shares at Dec. 31, 2015		10	8,919
Issuance of common stock, net of issuance costs	39,700	$ 0	$ 2	39,698	0
Issuance of common stock, net of issuance costs, shares		0	1,418
Exercise of stock options	$ 668	$ 0	$ 0	668	0
Exercise of stock options, shares	69	0	69
Issuance of restricted common stock	$ 0	$ 0	$ 0	0	0
Issuance of restricted common stock, shares		0	212
Forfeiture of restricted common stock	0	$ 0	$ 0	0	0
Forfeiture of restricted common stock, shares		0	(12)
Purchase and retirement of common stock	(2,537)	$ 0	$ 0	(2,537)	0
Purchase and retirement of common stock, shares		0	(83)
Stock-based compensation	5,717	$ 0	$ 0	5,717	0
Net loss	(21,672)	0	0	0	(21,672)
Balance at Dec. 31, 2016	73,334	$ 50	$ 11	120,985	(47,712)
Balance, shares at Dec. 31, 2016		10	10,523
Issuance of common stock, net of issuance costs	29,677	$ 0	$ 1	29,676	0
Issuance of common stock, net of issuance costs, shares		0	831
Exercise of stock options	$ 1,183	$ 0	$ 0	1,183	0
Exercise of stock options, shares	106	0	106
Issuance of restricted common stock	$ 0	$ 0	$ 0	0	0
Issuance of restricted common stock, shares		0	317
Forfeiture of restricted common stock	0	$ 0	$ 0	0	0
Forfeiture of restricted common stock, shares		0	(26)
Purchase and retirement of common stock	(3,030)	$ 0	$ 0	(3,030)	0
Purchase and retirement of common stock, shares		0	(100)
Stock-based compensation	6,954	$ 0	$ 0	6,979	(25)
Net loss	(25,771)	0	0	0	(25,771)
Balance at Dec. 31, 2017	$ 82,347	$ 50	$ 12	$ 155,793	$ (73,508)
Balance, shares at Dec. 31, 2017		10	11,651