Condensed Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss	Total
Balance at Dec. 31, 2017	$ 26	$ 641,801	$ (396,291)	$ (845)	$ 244,691
Balance (in shares) at Dec. 31, 2017	26,304,669
Increase (Decrease) in Stockholders' Equity
Exercise of stock options	$ 1	9,775			9,776
Exercise of stock options (in shares)	285,413
Exercise of warrants (in shares)	159,944
Vesting of restricted stock units (in shares)	1,562
Stock-based compensation		5,921			5,921
Other comprehensive gain (loss)				(118)	(118)
Net income (loss)			(46,130)		(46,130)
Balance at Mar. 31, 2018	$ 27	657,497	(442,421)	(963)	214,140
Balance (in shares) at Mar. 31, 2018	26,751,588
Balance at Dec. 31, 2017	$ 26	641,801	(396,291)	(845)	244,691
Balance (in shares) at Dec. 31, 2017	26,304,669
Increase (Decrease) in Stockholders' Equity
Net income (loss)					(141,771)
Balance at Sep. 30, 2018	$ 27	670,340	(538,062)	(560)	131,745
Balance (in shares) at Sep. 30, 2018	26,804,300
Balance at Mar. 31, 2018	$ 27	657,497	(442,421)	(963)	214,140
Balance (in shares) at Mar. 31, 2018	26,751,588
Increase (Decrease) in Stockholders' Equity
Exercise of stock options		1,597			1,597
Exercise of stock options (in shares)	49,504
Vesting of restricted stock units (in shares)	625
Stock-based compensation		5,723			5,723
Other comprehensive gain (loss)				187	187
Net income (loss)			(45,730)		(45,730)
Balance at Jun. 30, 2018	$ 27	664,817	(488,151)	(776)	175,917
Balance (in shares) at Jun. 30, 2018	26,801,717
Increase (Decrease) in Stockholders' Equity
Exercise of stock options		28			28
Exercise of stock options (in shares)	1,958
Vesting of restricted stock units (in shares)	625
Stock-based compensation		5,495			5,495
Other comprehensive gain (loss)				216	216
Net income (loss)			(49,911)		(49,911)
Balance at Sep. 30, 2018	$ 27	670,340	(538,062)	(560)	131,745
Balance (in shares) at Sep. 30, 2018	26,804,300
Balance at Dec. 31, 2018	$ 27	677,511	(598,101)	(319)	$ 79,118
Balance (in shares) at Dec. 31, 2018	26,824,859				26,824,859
Increase (Decrease) in Stockholders' Equity
Exercise of stock options		1,669			$ 1,669
Exercise of stock options (in shares)	80,218
Vesting of restricted stock units (in shares)	3,125
Stock-based compensation		6,636			6,636
Other comprehensive gain (loss)				208	208
Net income (loss)			87,379		87,379
Balance at Mar. 31, 2019	$ 27	685,816	(510,722)	(111)	175,010
Balance (in shares) at Mar. 31, 2019	26,908,202
Balance at Dec. 31, 2018	$ 27	677,511	(598,101)	(319)	$ 79,118
Balance (in shares) at Dec. 31, 2018	26,824,859				26,824,859
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)					333,708
Net income (loss)					$ (35,219)
Balance at Sep. 30, 2019	$ 27	701,794	(633,320)	11	$ 68,512
Balance (in shares) at Sep. 30, 2019	27,178,230				27,178,230
Balance at Mar. 31, 2019	$ 27	685,816	(510,722)	(111)	$ 175,010
Balance (in shares) at Mar. 31, 2019	26,908,202
Increase (Decrease) in Stockholders' Equity
Exercise of stock options		1,887			1,887
Exercise of stock options (in shares)	115,612
Exercise of warrants (in shares)	5,813
Vesting of restricted stock units (in shares)	7,025
Stock-based compensation		6,563			6,563
Other comprehensive gain (loss)				95	95
Net income (loss)			(54,221)		(54,221)
Balance at Jun. 30, 2019	$ 27	694,266	(564,943)	(16)	129,334
Balance (in shares) at Jun. 30, 2019	27,036,652
Increase (Decrease) in Stockholders' Equity
Exercise of stock options		1,201			1,201
Exercise of stock options (in shares)	137,878
Vesting of restricted stock units (in shares)	3,700
Stock-based compensation		6,327			6,327
Other comprehensive gain (loss)				27	27
Net income (loss)			(68,377)		(68,377)
Balance at Sep. 30, 2019	$ 27	$ 701,794	$ (633,320)	$ 11	$ 68,512
Balance (in shares) at Sep. 30, 2019	27,178,230				27,178,230