Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive (Loss) Income	Other Accumulated Deficit	Total
Balance, beginning of period at Dec. 31, 2018	$ 5	$ 652,464	$ (29)	$ (416,278)	$ 236,162
Balance, beginning of period (in shares) at Dec. 31, 2018	48,758,951
Statement of Stockholders' Equity (Deficit)
Exercise of stock options		1,533			1,533
Exercise of stock options (in shares)	146,171
Vesting of restricted common stock awards (in shares)	18,000
Stock-based compensation expense		7,855			7,855
Unrealized gain (loss) on marketable debt securities			58		58
Net loss				(29,249)	(29,249)
Balance, end of period at Mar. 31, 2019	$ 5	661,852	29	(444,724)	217,162
Balance, end of period (in shares) at Mar. 31, 2019	48,923,122
Balance, beginning of period at Dec. 31, 2018	$ 5	652,464	(29)	(416,278)	236,162
Balance, beginning of period (in shares) at Dec. 31, 2018	48,758,951
Statement of Stockholders' Equity (Deficit)
Net loss					(63,035)
Balance, end of period at Jun. 30, 2019	$ 5	671,522	45	(478,510)	193,062
Balance, end of period (in shares) at Jun. 30, 2019	49,241,093
Statement of Stockholders' Equity (Deficit)
Cumulative effect adjustment for adoption of new accounting guidance				803	803
Balance, beginning of period at Mar. 31, 2019	$ 5	661,852	29	(444,724)	217,162
Balance, beginning of period (in shares) at Mar. 31, 2019	48,923,122
Statement of Stockholders' Equity (Deficit)
Exercise of stock options		2,894			2,894
Exercise of stock options (in shares)	277,259
Vesting of restricted common stock awards		410			410
Vesting of restricted common stock awards (in shares)	24,486
Purchase of common stock under benefit plans		283			283
Purchase of common stock under benefit plans (in shares)	16,226
Stock-based compensation expense		6,083			6,083
Unrealized gain (loss) on marketable debt securities			16		16
Net loss				(33,786)	(33,786)
Balance, end of period at Jun. 30, 2019	$ 5	671,522	45	(478,510)	193,062
Balance, end of period (in shares) at Jun. 30, 2019	49,241,093
Balance, beginning of period at Dec. 31, 2019	$ 5	811,546	107	(549,221)	262,437
Balance, beginning of period (in shares) at Dec. 31, 2019	54,355,798
Statement of Stockholders' Equity (Deficit)
Exercise of stock options		3,047			3,047
Exercise of stock options (in shares)	233,208
Vesting of restricted common stock awards (in shares)	213,393
Stock-based compensation expense		6,220			6,220
Unrealized gain (loss) on marketable debt securities			587		587
Net loss				(37,724)	(37,724)
Balance, end of period at Mar. 31, 2020	$ 5	820,813	694	(586,945)	234,567
Balance, end of period (in shares) at Mar. 31, 2020	54,802,399
Balance, beginning of period at Dec. 31, 2019	$ 5	811,546	107	(549,221)	262,437
Balance, beginning of period (in shares) at Dec. 31, 2019	54,355,798
Statement of Stockholders' Equity (Deficit)
Net loss					(61,296)
Balance, end of period at Jun. 30, 2020	$ 6	1,037,100	183	(610,517)	426,772
Balance, end of period (in shares) at Jun. 30, 2020	62,103,649
Balance, beginning of period at Mar. 31, 2020	$ 5	820,813	694	(586,945)	234,567
Balance, beginning of period (in shares) at Mar. 31, 2020	54,802,399
Statement of Stockholders' Equity (Deficit)
Exercise of stock options		6,839			6,839
Exercise of stock options (in shares)	355,812
Vesting of restricted common stock awards (in shares)	30,194
Issuance of common stock for public offering	$ 1	203,681			203,682
Issuance of common stock for public offering (in shares)	6,900,000
Purchase of common stock under benefit plans		350			350
Purchase of common stock under benefit plans (in shares)	15,244
Stock-based compensation expense		5,417			5,417
Unrealized gain (loss) on marketable debt securities			(511)		(511)
Net loss				(23,572)	(23,572)
Balance, end of period at Jun. 30, 2020	$ 6	$ 1,037,100	$ 183	$ (610,517)	$ 426,772
Balance, end of period (in shares) at Jun. 30, 2020	62,103,649