STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit	Convertible Preferred Stock
Balance, Shares at Dec. 31, 2017		2,114
Balance at Dec. 31, 2017	$ (58,768)	$ 3	$ 1,803	$ 0	$ (60,574)
Temporary Equity, Balance, Shares at Dec. 31, 2017						30,971
Temporary Equity, Balance at Dec. 31, 2017						$ 142,969
Stock-based compensation expense	614	0	614	0	0	0
Vesting of restricted common stock	6	0	6	0	0	0
Net loss	(8,485)	$ 0	0	0	(8,485)	$ 0
Balance, Shares at Mar. 31, 2018		2,114
Balance at Mar. 31, 2018	(66,633)	$ 3	2,423	0	(69,059)
Temporary Equity, Balance, Shares at Mar. 31, 2018						30,971
Temporary Equity, Balance at Mar. 31, 2018						$ 142,969
Balance, Shares at Dec. 31, 2017		2,114
Balance at Dec. 31, 2017	(58,768)	$ 3	1,803	0	(60,574)
Temporary Equity, Balance, Shares at Dec. 31, 2017						30,971
Temporary Equity, Balance at Dec. 31, 2017						$ 142,969
Unrealized gain on marketable securities, net of tax	3
Net loss	(17,862)
Balance, Shares at Jun. 30, 2018		2,530
Balance at Jun. 30, 2018	(75,272)	$ 3	3,158	3	(78,436)
Temporary Equity, Balance, Shares at Jun. 30, 2018						30,971
Temporary Equity, Balance at Jun. 30, 2018						$ 143,019
Balance, Shares at Mar. 31, 2018		2,114
Balance at Mar. 31, 2018	(66,633)	$ 3	2,423	0	(69,059)
Temporary Equity, Balance, Shares at Mar. 31, 2018						30,971
Temporary Equity, Balance at Mar. 31, 2018						$ 142,969
Stock-based compensation expense	438	0	438	0	0	0
Proceeds from repayment of recourse promissory note						50
Issuance of common stock upon exercise of stock options	291	$ 0	291	0	0	0
Issuance of common stock upon exercise of stock options, Shares		416
Vesting of restricted common stock	6	$ 0	6	0	0	0
Unrealized gain on marketable securities, net of tax	3	0	0	3	0	0
Net loss	(9,377)	$ 0	0	0	(9,377)	$ 0
Balance, Shares at Jun. 30, 2018		2,530
Balance at Jun. 30, 2018	(75,272)	$ 3	3,158	3	(78,436)
Temporary Equity, Balance, Shares at Jun. 30, 2018						30,971
Temporary Equity, Balance at Jun. 30, 2018						$ 143,019
Balance, Shares at Dec. 31, 2018		42,117
Balance at Dec. 31, 2018	183,994	$ 42	288,079	(15)	(104,112)
Temporary Equity, Balance, Shares at Dec. 31, 2018						0
Temporary Equity, Balance at Dec. 31, 2018						$ 0
Stock-based compensation expense	2,834	0	2,834	0	0	0
Issuance of common stock upon exercise of stock options	368	$ 1	367	0	0	0
Issuance of common stock upon exercise of stock options, Shares		968
Issuance of common stock upon 2018 ESPP purchase	595	$ 0	595	0	0	0
Issuance of common stock upon 2018 ESPP purchase, Shares		39
Vesting of restricted common stock	6	$ 0	6	0	0	0
Unrealized gain on marketable securities, net of tax	45	0	0	45	0	0
Net loss	(19,953)	$ 0	0	0	(19,953)	$ 0
Balance, Shares at Mar. 31, 2019		43,124
Balance at Mar. 31, 2019	167,889	$ 43	291,881	30	(124,065)
Temporary Equity, Balance, Shares at Mar. 31, 2019						0
Temporary Equity, Balance at Mar. 31, 2019						$ 0
Balance, Shares at Dec. 31, 2018		42,117
Balance at Dec. 31, 2018	$ 183,994	$ 42	288,079	(15)	(104,112)
Temporary Equity, Balance, Shares at Dec. 31, 2018						0
Temporary Equity, Balance at Dec. 31, 2018						$ 0
Issuance of common stock upon exercise of stock options, Shares	1,029
Unrealized gain on marketable securities, net of tax	$ 129
Net loss	(39,025)
Balance, Shares at Jun. 30, 2019		43,184
Balance at Jun. 30, 2019	152,150	$ 43	295,130	114	(143,137)
Temporary Equity, Balance, Shares at Jun. 30, 2019						0
Temporary Equity, Balance at Jun. 30, 2019						$ 0
Balance, Shares at Mar. 31, 2019		43,124
Balance at Mar. 31, 2019	167,889	$ 43	291,881	30	(124,065)
Temporary Equity, Balance, Shares at Mar. 31, 2019						0
Temporary Equity, Balance at Mar. 31, 2019						$ 0
Stock-based compensation expense	3,105	0	3,105	0	0	0
Issuance of common stock upon exercise of stock options	138	$ 0	138	0	0	0
Issuance of common stock upon exercise of stock options, Shares		60
Vesting of restricted common stock	6	$ 0	6	0	0	0
Unrealized gain on marketable securities, net of tax	84	0	0	84	0	0
Net loss	(19,072)	$ 0	0	0	(19,072)	$ 0
Balance, Shares at Jun. 30, 2019		43,184
Balance at Jun. 30, 2019	$ 152,150	$ 43	$ 295,130	$ 114	$ (143,137)
Temporary Equity, Balance, Shares at Jun. 30, 2019						0
Temporary Equity, Balance at Jun. 30, 2019						$ 0