Unaudited Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Preferred Stock	Common Stock	Additional paid-in capital	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2018		0	41,397
Balance at beginning of period at Dec. 31, 2018	$ 123,959	$ 0	$ 41	$ 545,585	$ (421,667)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plans (in shares)			56
Issuance of common stock under employee stock compensation plans	41			41
Share-based compensation	2,470			2,470
Net loss	(12,501)				(12,501)
Balance at end of period (in shares) at Mar. 31, 2019		0	41,453
Balance at end of period at Mar. 31, 2019	113,969	$ 0	$ 41	548,096	(434,168)
Balance at beginning of period (in shares) at Dec. 31, 2019		0	49,627
Balance at beginning of period at Dec. 31, 2019	117,203	$ 0	$ 50	597,679	(480,526)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plans (in shares)			105
Issuance of common stock under employee stock compensation plans	179			179
Share-based compensation	2,324			2,324
Net loss	(15,076)				(15,076)
Balance at end of period (in shares) at Mar. 31, 2020		0	49,732
Balance at end of period at Mar. 31, 2020	$ 104,630	$ 0	$ 50	$ 600,182	$ (495,602)