Condensed Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at Dec. 31, 2018	$ 7	$ 607,236	$ (552)	$ (574,529)	$ 32,162
Balance (in shares) at Dec. 31, 2018	62,083
Cumulative-effect adjustment upon adoption		(185)		185
Issuance of common stock upon exercise of stock options		2,578			2,578
Issuance of common stock upon exercise of stock options (in shares)	1,070
Vesting of restricted stock (in shares)	187
Stock based compensation		4,051			4,051
Unrealized gain on available-for-sale securities			286		286
Net loss				(34,091)	(34,091)
Balance at Mar. 31, 2019	$ 7	613,680	(266)	(608,435)	4,986
Balance (in shares) at Mar. 31, 2019	63,340
Balance at Dec. 31, 2019	$ 8	976,955	919	(699,171)	$ 278,711
Balance (in shares) at Dec. 31, 2019	78,005				78,005
Cumulative-effect adjustment upon adoption				(403)	$ (403)
Issuance of common stock upon exercise of stock options		3,826			3,826
Issuance of common stock upon exercise of stock options (in shares)	356
Vesting of restricted stock (in shares)	291
Stock based compensation		7,418			7,418
Unrealized gain on available-for-sale securities			4,747		4,747
Net loss				(35,372)	(35,372)
Balance at Mar. 31, 2020	$ 8	$ 988,199	$ 5,666	$ (734,946)	$ 258,927
Balance (in shares) at Mar. 31, 2020	78,652				78,652