Consolidated Statements of Stockholders' Equity (unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2017	$ 55,991	$ 28	$ 164,995	$ (109,252)	$ 220
Balance, shares at Dec. 31, 2017		28,330,460
Cumulative effect of adoption of ASU 2014-09, net of taxes	9,097			9,097
Stock-based compensation	6,694		6,694
Vesting of restricted stock units, shares		222,624
Restricted stock units withheld to settle employee tax withholding liability	(1,022)		(1,022)
Restricted stock units withheld to settle employee tax withholding liability, shares		(27,771)
Exercise of stock options	1,466	$ 1	1,465
Exercise of stock options, shares		153,744
Issuance of shares under employee stock purchase plan	881		881
Issuance of shares under employee stock purchase plan, shares		44,193
Other comprehensive income	(267)				(267)
Net loss	(12,342)			(12,342)
Balance at Mar. 31, 2018	60,498	$ 29	173,013	(112,497)	(47)
Balance, shares at Mar. 31, 2018		28,723,250
Balance at Dec. 31, 2017	$ 55,991	$ 28	164,995	(109,252)	220
Balance, shares at Dec. 31, 2017		28,330,460
Exercise of stock options, shares	778,370
Balance at Dec. 31, 2018	$ 42,158	$ 30	194,866	(147,670)	(5,068)
Balance, shares at Dec. 31, 2018		29,700,192
Issuance of common stock, net of cost	138,839	$ 3	138,836
Issuance of common stock, net of cost, shares		2,645,000
Stock-based compensation	7,849		7,849
Vesting of restricted stock units, shares		16,849
Restricted stock units withheld to settle employee tax withholding liability	(333)		(333)
Restricted stock units withheld to settle employee tax withholding liability, shares		(5,301)
Exercise of stock options	$ 8,746		8,746
Exercise of stock options, shares	501,083	501,083
Issuance of shares under employee stock purchase plan	$ 1,283		1,283
Issuance of shares under employee stock purchase plan, shares		24,266
Other comprehensive income	22				22
Net loss	(14,134)			(14,134)
Balance at Mar. 31, 2019	$ 184,430	$ 33	$ 351,247	$ (161,804)	$ (5,046)
Balance, shares at Mar. 31, 2019		32,882,089