Condensed 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 (loss)	(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
Net loss	(29,260)
Balance at Jun. 30, 2018	52,837	$ 29	184,654	(129,415)	(2,431)
Balance, shares at Jun. 30, 2018		29,259,367
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
Balance at Mar. 31, 2018	60,498	$ 29	173,013	(112,497)	(47)
Balance, shares at Mar. 31, 2018		28,723,250
Stock-based compensation	10,022		10,022
Vesting of restricted stock units, shares		221,624
Restricted stock units withheld to settle employee tax withholding liability	(2,750)		(2,750)
Restricted stock units withheld to settle employee tax withholding liability, shares		(53,326)
Exercise of stock options	4,369		4,369
Exercise of stock options, shares		367,819
Other comprehensive income (loss)	(2,384)				(2,384)
Net loss	(16,918)			(16,918)
Balance at Jun. 30, 2018	52,837	$ 29	184,654	(129,415)	(2,431)
Balance, shares at Jun. 30, 2018		29,259,367
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
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 (loss)	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
Balance at Dec. 31, 2018	$ 42,158	$ 30	194,866	(147,670)	(5,068)
Balance, shares at Dec. 31, 2018		29,700,192
Exercise of stock options, shares	745,003
Net loss	$ (26,197)
Balance at Jun. 30, 2019	185,540	$ 33	364,149	(173,867)	(4,775)
Balance, shares at Jun. 30, 2019		33,150,319
Balance at Mar. 31, 2019	184,430	$ 33	351,247	(161,804)	(5,046)
Balance, shares at Mar. 31, 2019		32,882,089
Issuance of common stock, net of cost	(5)		(5)
Stock-based compensation	8,282		8,282
Vesting of restricted stock units, shares		25,720
Restricted stock units withheld to settle employee tax withholding liability	(116)		(116)
Restricted stock units withheld to settle employee tax withholding liability, shares		(1,410)
Exercise of stock options	4,741		4,741
Exercise of stock options, shares		243,920
Other comprehensive income (loss)	271				271
Net loss	(12,063)			(12,063)
Balance at Jun. 30, 2019	$ 185,540	$ 33	$ 364,149	$ (173,867)	$ (4,775)
Balance, shares at Jun. 30, 2019		33,150,319