Condensed Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Beginning Balances at Dec. 31, 2017	$ 82,753	$ 1	$ 456,984	$ (374,232)
Beginning Balances, shares at Dec. 31, 2017		5,989,645
Issuance of common stock upon exercise of options	35		35
Issuance of common stock upon exercise of options, shares		3,643
Issuance of common stock under employee benefit plans, shares		18,300
Stock-based compensation	2,820		2,820
Net loss	(8,981)			(8,981)
Ending Balances at Mar. 31, 2018	76,627	$ 1	459,839	(383,213)
Ending Balances, shares at Mar. 31, 2018		6,011,588
Beginning Balances at Dec. 31, 2017	82,753	$ 1	456,984	(374,232)
Beginning Balances, shares at Dec. 31, 2017		5,989,645
Net loss	(18,829)
Ending Balances at Jun. 30, 2018	68,934	$ 1	461,994	(393,061)
Ending Balances, shares at Jun. 30, 2018		6,040,065
Beginning Balances at Mar. 31, 2018	76,627	$ 1	459,839	(383,213)
Beginning Balances, shares at Mar. 31, 2018		6,011,588
Issuance of common stock under employee benefit plans, shares		28,477
Stock-based compensation	2,155		2,155
Net loss	(9,848)			(9,848)
Ending Balances at Jun. 30, 2018	68,934	$ 1	461,994	(393,061)
Ending Balances, shares at Jun. 30, 2018		6,040,065
Beginning Balances at Dec. 31, 2018	59,945	$ 1	510,509	(450,565)
Beginning Balances, shares at Dec. 31, 2018		11,266,151
Issuance of common stock under employee benefit plans, shares		10,349
Stock-based compensation	1,048		1,048
Cumulative-effect adjustment to equity due to adoption of ASU 2016-02	(1,328)			(1,328)
Net loss	(4,704)			(4,704)
Ending Balances at Mar. 31, 2019	54,961	$ 1	511,557	(456,597)
Ending Balances, shares at Mar. 31, 2019		11,276,500
Beginning Balances at Dec. 31, 2018	$ 59,945	$ 1	510,509	(450,565)
Beginning Balances, shares at Dec. 31, 2018		11,266,151
Issuance of common stock upon exercise of options, shares	2,000
Net loss	$ (7,748)
Ending Balances at Jun. 30, 2019	52,871	$ 1	512,511	(459,641)
Ending Balances, shares at Jun. 30, 2019		11,284,580
Beginning Balances at Mar. 31, 2019	54,961	$ 1	511,557	(456,597)
Beginning Balances, shares at Mar. 31, 2019		11,276,500
Issuance of common stock upon exercise of options	2		2
Issuance of common stock upon exercise of options, shares		2,000
Issuance of common stock under employee benefit plans	11		11
Issuance of common stock under employee benefit plans, shares		6,080
Stock-based compensation	941		941
Net loss	(3,044)			(3,044)
Ending Balances at Jun. 30, 2019	$ 52,871	$ 1	$ 512,511	$ (459,641)
Ending Balances, shares at Jun. 30, 2019		11,284,580