Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity - USD ($) $ in Thousands	Total		Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss
Balance at Dec. 31, 2017	$ (72,328)				$ 948	$ (73,234)	$ (42)
Balance, shares at Dec. 31, 2017			30,459,574
Balance at Dec. 31, 2017			$ 226,196
Balance, shares at Dec. 31, 2017				3,278,129
Conversion of preferred stock to common stock	226,198			$ 3	226,195
Conversion of preferred stock to common stock, shares			(30,459,574)
Conversion of preferred stock to common stock			$ (226,196)
Conversion of preferred stock to common stock, shares				30,459,574
Issuance of common stock upon IPO	124,716			$ 1	124,715
Issuance of common stock upon IPO, shares				9,200,000
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares	80				80
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares				60,524
Vesting of early exercised stock options and restricted stock	278				278
Vesting of early exercised stock options and restricted stock, shares				129,152
Stock-based compensation	656				656
Other comprehensive income (loss)	(55)						(55)
Net loss	(12,955)					(12,955)
Balance at Mar. 31, 2018	266,590			$ 4	352,872	(86,189)	(97)
Balance, shares at Mar. 31, 2018				43,127,379
Balance at Dec. 31, 2017	$ (72,328)				948	(73,234)	(42)
Balance, shares at Dec. 31, 2017			30,459,574
Balance at Dec. 31, 2017			$ 226,196
Balance, shares at Dec. 31, 2017				3,278,129
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares	713,931
Net loss	$ (26,488)
Balance at Jun. 30, 2018	254,574			$ 4	354,375	(99,722)	(83)
Balance, shares at Jun. 30, 2018				43,276,717
Balance at Mar. 31, 2018	$ 266,590			$ 4	352,872	(86,189)	(97)
Balance, shares at Mar. 31, 2018				43,127,379
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares	5,171
Vesting of early exercised stock options and restricted stock	$ 387				387
Vesting of early exercised stock options and restricted stock, shares				149,338
Stock-based compensation	1,116				1,116
Other comprehensive income (loss)	14						14
Net loss	(13,533)					(13,533)
Balance at Jun. 30, 2018	254,574			$ 4	354,375	(99,722)	(83)
Balance, shares at Jun. 30, 2018				43,276,717
Balance at Dec. 31, 2018	$ 234,942	[1]		$ 4	357,873	(122,828)	(107)
Balance, shares at Dec. 31, 2018	44,537,946			43,610,823
Cumulative effect adjustment upon adoption of ASC 606 | ASC 606	$ 2,212					2,212
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares				69
Vesting of early exercised stock options and restricted stock	273				273
Vesting of early exercised stock options and restricted stock, shares				114,934
Stock-based compensation	1,674				1,674
Other comprehensive income (loss)	136						136
Net loss	(17,670)					(17,670)
Balance at Mar. 31, 2019	221,567			$ 4	359,820	(138,286)	29
Balance, shares at Mar. 31, 2019				43,725,826
Balance at Dec. 31, 2018	$ 234,942	[1]		$ 4	357,873	(122,828)	(107)
Balance, shares at Dec. 31, 2018	44,537,946			43,610,823
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares	18,770
Net loss	$ (45,760)
Balance at Jun. 30, 2019	$ 196,646			$ 4	362,905	(166,376)	113
Balance, shares at Jun. 30, 2019	45,854,213			43,934,108
Balance at Mar. 31, 2019	$ 221,567			$ 4	359,820	(138,286)	29
Balance, shares at Mar. 31, 2019				43,725,826
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares	$ 99				99
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares	18,701			17,649
Vesting of early exercised stock options and restricted stock	$ 260				260
Vesting of early exercised stock options and restricted stock, shares				107,952
Issuance of common stock under Employee Stock Purchase Plan	596				596
Issuance of common stock under Employee Stock Purchase Plan, shares				82,681
Stock-based compensation	2,130				2,130
Other comprehensive income (loss)	84						84
Net loss	(28,090)					(28,090)
Balance at Jun. 30, 2019	$ 196,646			$ 4	$ 362,905	$ (166,376)	$ 113
Balance, shares at Jun. 30, 2019	45,854,213			43,934,108

[1] The Condensed Consolidated Balance Sheet as of December 31, 2018 has been derived from the audited financial statements as of that date.