Condensed Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	AOCI Attributable to Parent	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		44,616,586
Beginning balance at Dec. 31, 2017	$ 242,036	$ 4	$ 1,124,630	$ (314)	$ (882,284)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(68,882)				(68,882)
Unrealized loss from available-for-sale securities	193			193
Vesting of restricted shares (in shares)		8,261
Exercise of options (in shares)		31,062
Exercise of options	2,250		2,250
Exercise of warrants (in shares)		222,745
Share-based compensation expense related to share-based awards for employee stock purchase plan	252		252
Share-based compensation expense	7,768		7,768
Ending balance (in shares) at Mar. 31, 2018		45,214,387
Ending balance at Mar. 31, 2018	195,178	$ 5	1,140,495	(1,483)	(943,839)
Beginning balance (in shares) at Dec. 31, 2017		44,616,586
Beginning balance at Dec. 31, 2017	242,036	$ 4	1,124,630	(314)	(882,284)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(130,437)				(130,437)
Unrealized loss from available-for-sale securities	(976)			(976)
Vesting of restricted shares (in shares)		24,372
Exercise of options (in shares)		444,748
Exercise of options	8,826	$ 1	8,825
Exercise of warrants (in shares)		336,059
Share-based compensation expense related to share-based awards for employee stock purchase plan	414		414
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		54,690
Issuance of common stock upon purchase by employee stock purchase plan	1,741		1,741
Share-based compensation expense	15,155		15,155
Ending balance (in shares) at Jun. 30, 2018		45,476,455
Ending balance at Jun. 30, 2018	136,759	$ 5	1,150,765	(1,290)	(1,012,721)
Beginning balance (in shares) at Mar. 31, 2018		45,214,387
Beginning balance at Mar. 31, 2018	195,178	$ 5	1,140,495	(1,483)	(943,839)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(68,882)
Unrealized loss from available-for-sale securities	192
Ending balance (in shares) at Jun. 30, 2018		45,476,455
Ending balance at Jun. 30, 2018	136,759	$ 5	1,150,765	(1,290)	(1,012,721)
Beginning balance (in shares) at Dec. 31, 2018		45,563,693
Beginning balance at Dec. 31, 2018	60,632	$ 5	1,165,003	(755)	(1,103,621)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(78,234)				(78,234)
Unrealized loss from available-for-sale securities	710			710
Vesting of restricted shares (in shares)		75,331
Exercise of options (in shares)		341,337
Exercise of options	2,869		2,869
Exercise of warrants (in shares)		81,104
Exercise of warrants	1,000		1,000
Share-based compensation expense related to share-based awards for employee stock purchase plan	436		436
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		63,732
Issuance of common stock upon purchase by employee stock purchase plan	1,027		1,027
Share-based compensation expense	11,426		11,426
Ending balance (in shares) at Jun. 30, 2019		46,125,197
Ending balance at Jun. 30, 2019	(134)	$ 5	1,181,761	(45)	(1,181,855)
Beginning balance (in shares) at Mar. 31, 2019		45,967,080
Beginning balance at Mar. 31, 2019	28,004	$ 5	1,174,661	(281)	(1,146,381)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(35,474)				(35,474)
Unrealized loss from available-for-sale securities	236			236
Vesting of restricted shares (in shares)		2,218
Exercise of options (in shares)		155,899
Exercise of options	1,352		1,352
Share-based compensation expense related to share-based awards for employee stock purchase plan	255		255
Share-based compensation expense	5,493		5,493
Ending balance (in shares) at Jun. 30, 2019		46,125,197
Ending balance at Jun. 30, 2019	$ (134)	$ 5	$ 1,181,761	$ (45)	$ (1,181,855)