Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ 183,973	$ 0	$ 326	$ 379,516	$ 0	$ (195,869)
Beginning balance (in shares) at Dec. 31, 2017		0	32,591,686
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs	185,259		$ 50	185,209
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares)			4,945,000
Issuance of common stock upon exercise of stock options	808		$ 1	807
Issuance of common stock upon exercise of stock options (in shares)			124,515
Stock-based compensation expense	6,883			6,883
Net loss	(67,555)					(67,555)
Ending balance at Sep. 30, 2018	309,368	$ 0	$ 377	572,415	0	(263,424)
Ending balance (in shares) at Sep. 30, 2018		0	37,661,201
Beginning balance at Dec. 31, 2017	183,973	$ 0	$ 326	379,516	0	(195,869)
Beginning balance (in shares) at Dec. 31, 2017		0	32,591,686
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(99,900)
Ending balance at Dec. 31, 2018	279,981	$ 0	$ 377	575,327	0	(295,723)
Ending balance (in shares) at Dec. 31, 2018		0	37,676,760
Beginning balance at Jun. 30, 2018	331,097	$ 0	$ 376	569,710	0	(238,989)
Beginning balance (in shares) at Jun. 30, 2018		0	37,633,450
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options	92		$ 1	91
Issuance of common stock upon exercise of stock options (in shares)			27,751
Stock-based compensation expense	2,614			2,614
Net loss	(24,435)					(24,435)
Ending balance at Sep. 30, 2018	309,368	$ 0	$ 377	572,415	0	(263,424)
Ending balance (in shares) at Sep. 30, 2018		0	37,661,201
Beginning balance at Dec. 31, 2018	279,981	$ 0	$ 377	575,327	0	(295,723)
Beginning balance (in shares) at Dec. 31, 2018		0	37,676,760
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs	431,780		$ 124	431,656
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares)			12,432,431
Issuance of common stock upon exercise of stock options	3,706		$ 9	3,697
Issuance of common stock upon exercise of stock options (in shares)			934,721
Stock-based compensation expense	15,065			15,065
Unrealized gains (losses) on marketable securities	61				61
Net loss	(125,040)					(125,040)
Ending balance at Sep. 30, 2019	605,553	$ 0	$ 510	1,025,745	61	(420,763)
Ending balance (in shares) at Sep. 30, 2019		0	51,043,912
Beginning balance at Jun. 30, 2019	222,878	$ 0	$ 382	586,888	175	(364,567)
Beginning balance (in shares) at Jun. 30, 2019		0	38,215,108
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs	431,780		$ 124	431,656
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares)			12,432,431
Issuance of common stock upon exercise of stock options	2,476		$ 4	2,472
Issuance of common stock upon exercise of stock options (in shares)			396,373
Stock-based compensation expense	4,729			4,729
Unrealized gains (losses) on marketable securities	(114)				(114)
Net loss	(56,196)					(56,196)
Ending balance at Sep. 30, 2019	$ 605,553	$ 0	$ 510	$ 1,025,745	$ 61	$ (420,763)
Ending balance (in shares) at Sep. 30, 2019		0	51,043,912