Condensed Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in-Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2017	$ 260,274	$ 38	$ 609,810	$ (407)	$ (349,167)
Balance (in shares) at Dec. 31, 2017		37,611
Issuance of common stock for equity awards	414		414
Issuance of common stock for equity awards (in shares)		22
Stock-based compensation expense	3,657		3,657
Net loss	(41,569)				(41,569)
Other comprehensive income (loss)	(169)			(169)
Balance at Mar. 31, 2018	222,607	$ 38	613,881	(576)	(390,736)
Balance (in shares) at Mar. 31, 2018		37,633
Balance at Dec. 31, 2017	260,274	$ 38	609,810	(407)	(349,167)
Balance (in shares) at Dec. 31, 2017		37,611
Net loss	(85,444)
Other comprehensive income (loss)	97
Balance at Jun. 30, 2018	185,507	$ 38	620,390	(310)	(434,611)
Balance (in shares) at Jun. 30, 2018		37,808
Balance at Mar. 31, 2018	222,607	$ 38	613,881	(576)	(390,736)
Balance (in shares) at Mar. 31, 2018		37,633
Issuance of common stock for equity awards	1,396		1,396
Issuance of common stock for equity awards (in shares)		122
Employee stock purchase plan	1,129		1,129
Employee Stock Purchase Plan (in shares)		53
Stock-based compensation expense	3,984		3,984
Net loss	(43,875)				(43,875)
Other comprehensive income (loss)	266			266
Balance at Jun. 30, 2018	185,507	$ 38	620,390	(310)	(434,611)
Balance (in shares) at Jun. 30, 2018		37,808
Balance at Dec. 31, 2018	110,079	$ 38	628,944	(77)	(518,826)
Balance (in shares) at Dec. 31, 2018		37,946
Issuance of common stock for equity awards (in shares)		47
Stock-based compensation expense	3,853		3,853
Net loss	(41,538)				(41,538)
Other comprehensive income (loss)	182			182
Balance at Mar. 31, 2019	72,576	$ 38	632,797	105	(560,364)
Balance (in shares) at Mar. 31, 2019		37,993
Balance at Dec. 31, 2018	110,079	$ 38	628,944	(77)	(518,826)
Balance (in shares) at Dec. 31, 2018		37,946
Net loss	(78,025)
Other comprehensive income (loss)	307
Balance at Jun. 30, 2019	41,471	$ 38	638,054	230	(596,851)
Balance (in shares) at Jun. 30, 2019		38,107
Balance at Mar. 31, 2019	72,576	$ 38	632,797	105	(560,364)
Balance (in shares) at Mar. 31, 2019		37,993
Issuance of common stock for equity awards (in shares)		8
Employee stock purchase plan	1,040		1,040
Employee Stock Purchase Plan (in shares)		106
Stock-based compensation expense	4,217		4,217
Net loss	(36,487)				(36,487)
Other comprehensive income (loss)	125			125
Balance at Jun. 30, 2019	$ 41,471	$ 38	$ 638,054	$ 230	$ (596,851)
Balance (in shares) at Jun. 30, 2019		38,107