Consolidated Statements Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2017	$ 984	$ 714,869	$ (243)	$ (631,843)	$ 83,767
Impact to retained earnings from adoption of new standard at Dec. 31, 2017				1,126	1,126
Net loss				(25,777)	(25,777)
Other comprehensive income			(233)		(233)
Exercise of stock options	2	79			81
Employee stock purchase plan sales	1	186			187
Stock-based compensation expense		2,903			2,903
Balance at Mar. 31, 2018	987	718,037	(476)	(656,494)	62,054
Balance at Dec. 31, 2017	984	714,869	(243)	(631,843)	83,767
Impact to retained earnings from adoption of new standard at Dec. 31, 2017				1,126	1,126
Net loss					(44,223)
Balance at Jun. 30, 2018	989	720,587	(452)	(674,940)	46,184
Balance at Mar. 31, 2018	987	718,037	(476)	(656,494)	62,054
Net loss				(18,446)	(18,446)
Other comprehensive income			24		24
Exercise of stock options	2	466			468
Stock-based compensation expense		2,084			2,084
Balance at Jun. 30, 2018	989	720,587	(452)	(674,940)	46,184
Balance at Dec. 31, 2018	1,101	780,400	(297)	(731,969)	49,235
Impact to retained earnings from adoption of new standard at Dec. 31, 2018				238	238
Net loss				(31,054)	(31,054)
Other comprehensive income			208		208
Exercise of stock options	2	341			343
Employee stock purchase plan sales		220			220
Stock-based compensation expense		3,317			3,317
Balance at Mar. 31, 2019	1,103	784,278	(89)	(762,785)	22,507
Balance at Dec. 31, 2018	1,101	780,400	(297)	(731,969)	49,235
Impact to retained earnings from adoption of new standard at Dec. 31, 2018				238	238
Net loss					(68,683)
Balance at Jun. 30, 2019	1,104	790,076	41	(800,414)	(9,193)
Balance at Mar. 31, 2019	1,103	784,278	(89)	(762,785)	22,507
Net loss				(37,629)	(37,629)
Other comprehensive income			130		130
Exercise of stock options	1	413			414
Stock-based compensation expense		5,385			5,385
Balance at Jun. 30, 2019	$ 1,104	$ 790,076	$ 41	$ (800,414)	$ (9,193)