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, 2017		0	36,110
Balance at beginning of period at Dec. 31, 2017	$ 38,041	$ 0	$ 36	$ 522,759	$ (484,754)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plans (in shares)			54
Issuance of common stock under employee stock compensation plans	27			27
Share-based compensation	3,082			3,082
Net loss	(13,073)				(13,073)
Balance at end of period (in shares) at Mar. 31, 2018		0	36,164
Balance at end of period at Mar. 31, 2018	28,077	$ 0	$ 36	525,868	(497,827)
Balance at beginning of period (in shares) at Dec. 31, 2017		0	36,110
Balance at beginning of period at Dec. 31, 2017	38,041	$ 0	$ 36	522,759	(484,754)
Increase (Decrease) in Stockholders' Equity
Net loss	(41,027)
Balance at end of period (in shares) at Sep. 30, 2018		0	36,218
Balance at end of period at Sep. 30, 2018	5,427	$ 0	$ 36	531,172	(525,781)
Balance at beginning of period (in shares) at Mar. 31, 2018		0	36,164
Balance at beginning of period at Mar. 31, 2018	28,077	$ 0	$ 36	525,868	(497,827)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plans (in shares)			24
Issuance of common stock under employee stock compensation plans	0
Share-based compensation	2,662			2,662
Net loss	(13,209)				(13,209)
Balance at end of period (in shares) at Jun. 30, 2018		0	36,188
Balance at end of period at Jun. 30, 2018	17,530	$ 0	$ 36	528,530	(511,036)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plans (in shares)			30
Issuance of common stock under employee stock compensation plans	38			38
Share-based compensation	2,604			2,604
Net loss	(14,745)				(14,745)
Balance at end of period (in shares) at Sep. 30, 2018		0	36,218
Balance at end of period at Sep. 30, 2018	5,427	$ 0	$ 36	531,172	(525,781)
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, 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
Net loss	(41,381)
Balance at end of period (in shares) at Sep. 30, 2019		0	41,602
Balance at end of period at Sep. 30, 2019	89,462	$ 0	$ 42	552,468	(463,048)
Balance at beginning of period (in shares) at Mar. 31, 2019		0	41,453
Balance at beginning of period at Mar. 31, 2019	113,969	$ 0	$ 41	548,096	(434,168)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plans (in shares)			24
Issuance of common stock under employee stock compensation plans	0
Share-based compensation	2,207			2,207
Net loss	(14,443)				(14,443)
Balance at end of period (in shares) at Jun. 30, 2019		0	41,477
Balance at end of period at Jun. 30, 2019	101,733	$ 0	$ 41	550,303	(448,611)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock compensation plans (in shares)			50
Issuance of common stock under employee stock compensation plans	40		$ 1	39
Share-based compensation	2,126			2,126
Issuance of common stock (in shares)			75
Net loss	(14,437)				(14,437)
Balance at end of period (in shares) at Sep. 30, 2019		0	41,602
Balance at end of period at Sep. 30, 2019	$ 89,462	$ 0	$ 42	$ 552,468	$ (463,048)