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, 2015	$ 103,986	$ 22	$ 243,853	$ (97)	$ (139,792)
Balance (in shares) at Dec. 31, 2015		21,570
Issuance of common stock net of issuance costs	147,501	$ 10	147,491
Issuance of common stock net of issuance costs (in shares)		10,040
Issuance of common stock for equity awards	167		167
Issuance of common stock for equity awards (in shares)		30
Employee stock purchase plan	476		476
Employee Stock Purchase Plan (in shares)		27
Stock-based compensation expense	6,770		6,770
Net loss	(71,894)				(71,894)
Other comprehensive income (loss)	26			26
Balance at Dec. 31, 2016	187,032	$ 32	398,757	(71)	(211,686)
Balance (in shares) at Dec. 31, 2016		31,667
Issuance of common stock net of issuance costs	132,177	$ 6	132,171
Issuance of common stock net of issuance costs (in shares)		5,520
Issuance of common stock for equity awards	3,858		3,858
Issuance of common stock for equity awards (in shares)		334
Employee stock purchase plan	1,016		1,016
Employee Stock Purchase Plan (in shares)		90
Stock-based compensation expense	11,542		11,542
Portion of convertible debt proceeds allocated to equity component	62,466		62,466
Net loss	(137,481)			0	(137,481)
Other comprehensive income (loss)	(336)			(336)
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	1,810		1,810
Issuance of common stock for equity awards (in shares)		144
Employee stock purchase plan	1,129		1,129
Employee Stock Purchase Plan (in shares)		53
Stock-based compensation expense	7,641		7,641
Net loss	(85,444)				(85,444)
Other comprehensive income (loss)	97			97
Balance at Jun. 30, 2018	$ 185,507	$ 38	$ 620,390	$ (310)	$ (434,611)
Balance (in shares) at Jun. 30, 2018		37,808