Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	February 2015	July 2015	Common Stock	Common Stock February 2015	Common Stock July 2015	Additional Paid-in Capital	Additional Paid-in Capital February 2015	Additional Paid-in Capital July 2015	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2014	$ 103,182			$ 2			$ 144,169			$ (100)	$ (40,889)
Beginning balance (in shares) at Dec. 31, 2014				19,693,000
Issuance of common stock, net of discounts and offering costs, Value		$ 69,487	$ 193,947		$ 1			$ 69,486	$ 193,947
Issuance of common stock, net of discounts and offering costs, shares					4,147,000	3,981,000
Issuance of common stock upon vesting of restricted stock awards	80						80
Issuance of common stock upon vesting of restricted stock awards, shares				287,000
RSU settlements, net of shares withheld	(4,647)						(4,647)
RSU settlements, net of shares withheld, shares				327,000
Issuance of common stock pursuant to stock option exercises	$ 439						439
Issuance of common stock pursuant to stock option exercises, shares	23,822			24,000
Stock-based compensation expense	$ 10,251						10,251
Net loss	(57,221)										(57,221)
Unrealized gain (loss) on available-for-sale securities	(418)									(418)
Ending balance at Dec. 31, 2015	315,100			$ 3			413,725			(518)	(98,110)
Ending balance (in shares) at Dec. 31, 2015				28,459,000
Issuance of common stock upon vesting of restricted stock awards	60						60
Issuance of common stock upon vesting of restricted stock awards, shares				233,000
RSU settlements, net of shares withheld	(94)						(94)
RSU settlements, net of shares withheld, shares				199,000
Issuance of common stock pursuant to employee stock awards	$ 600						600
Issuance of common stock pursuant to employee stock awards, shares				42,000
Issuance of common stock pursuant to stock option exercises, shares	18,947
Stock-based compensation expense	$ 16,784						16,784
Net loss	(79,049)										(79,049)
Unrealized gain (loss) on available-for-sale securities	335									335
Ending balance at Dec. 31, 2016	253,736			$ 3			431,075			(183)	(177,159)
Ending balance (in shares) at Dec. 31, 2016				28,933,000
Issuance of common stock through ATM facility, net of commissions and offering costs, Value	19,156						19,156
Issuance of common stock through ATM facility, net of commissions and offering costs, Shares				1,350,000
RSU settlements, net of shares withheld	(357)						(357)
RSU settlements, net of shares withheld, shares				305,000
Issuance of common stock pursuant to employee stock awards	$ 1,688						1,688
Issuance of common stock pursuant to employee stock awards, shares				142,000
Issuance of common stock pursuant to stock option exercises, shares	60,125
Stock-based compensation expense	$ 23,100						23,100
Net loss	(119,491)										(119,491)
Unrealized gain (loss) on available-for-sale securities	32									32
Ending balance at Dec. 31, 2017	$ 177,864			$ 3			$ 474,662			$ (151)	$ (296,650)
Ending balance (in shares) at Dec. 31, 2017				30,730,000