CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive (Loss)/Gain	Accumulated deficit
Beginning balance at Dec. 31, 2017	$ 177,805	$ 5	$ 443,390	$ (108)	$ (265,482)
Beginning balances (in shares) at Dec. 31, 2017		51,091
Issuance of common stock upon exercise of vested options	2,743		2,743
Issuance of common stock upon exercise of vested options (in shares)		308
Issuance of common stock upon securities agreement (in shares)		300
Issuance of common stock upon public offering	190,454	$ 1	190,453
Issuance common stock upon initial public offering (in shares)		6,325
Stock-based compensation expense	7,607		7,607
Other comprehensive gain (loss)	(17)			(17)
Net loss	(49,500)				(49,500)
Ending balances at Mar. 31, 2018	329,092	$ 6	644,193	(125)	(314,982)
Ending balances (in shares) at Mar. 31, 2018		58,024
Beginning balance at Dec. 31, 2017	177,805	$ 5	443,390	(108)	(265,482)
Beginning balances (in shares) at Dec. 31, 2017		51,091
Other comprehensive gain (loss)	(15)
Net loss	(102,052)
Ending balances at Jun. 30, 2018	287,838	$ 6	655,490	(123)	(367,535)
Ending balances (in shares) at Jun. 30, 2018		58,461
Beginning balance at Mar. 31, 2018	329,092	$ 6	644,193	(125)	(314,982)
Beginning balances (in shares) at Mar. 31, 2018		58,024
Issuance of common stock upon exercise of vested options and vesting of restricted stock units	2,641		2,641
Issuance of common stock upon exercise of vested options and vesting of restricted stock units (in shares)		437
Issuance of common stock upon public offering	(19)		(18)		(1)
Stock-based compensation expense	8,674		8,674
Other comprehensive gain (loss)	2			2
Net loss	(52,552)				(52,552)
Ending balances at Jun. 30, 2018	287,838	$ 6	655,490	(123)	(367,535)
Ending balances (in shares) at Jun. 30, 2018		58,461
Beginning balance at Dec. 31, 2018	298,947	$ 6	775,283	(108)	(476,234)
Beginning balances (in shares) at Dec. 31, 2018		62,142
Issuance of common stock upon exercise of vested options	3,633		3,633
Issuance of common stock upon exercise of vested options (in shares)		328
Stock-based compensation expense	7,765		7,765
Other comprehensive gain (loss)	171			171
Accumulated depreciation upon adoption of ASU Topic 842	51				51
Net loss	(54,266)				(54,266)
Ending balances at Mar. 31, 2019	256,301	$ 6	786,681	63	(530,449)
Ending balances (in shares) at Mar. 31, 2019		62,470
Beginning balance at Dec. 31, 2018	298,947	$ 6	775,283	(108)	(476,234)
Beginning balances (in shares) at Dec. 31, 2018		62,142
Other comprehensive gain (loss)	345
Net loss	(117,144)
Ending balances at Jun. 30, 2019	204,974	$ 6	798,058	236	(593,326)
Ending balances (in shares) at Jun. 30, 2019		62,659
Beginning balance at Mar. 31, 2019	256,301	$ 6	786,681	63	(530,449)
Beginning balances (in shares) at Mar. 31, 2019		62,470
Issuance of common stock upon exercise of vested options and vesting of restricted stock units	2,637		2,637
Issuance of common stock upon exercise of vested options and vesting of restricted stock units (in shares)		189
Stock-based compensation expense	8,740		8,740
Other comprehensive gain (loss)	174			173	1
Net loss	(62,878)				(62,878)
Ending balances at Jun. 30, 2019	$ 204,974	$ 6	$ 798,058	$ 236	$ (593,326)
Ending balances (in shares) at Jun. 30, 2019		62,659