Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2014	$ 86,169	$ 3	$ 228,151		$ (141,985)
Beginning Balance, shares at Dec. 31, 2014		28,223,958
Exercise of stock options	4,739		4,739
Exercise of stock options, shares		1,070,457
Vesting of restricted stock	27		27
Vesting of restricted stock, shares		219,430
Issuance of common stock from Roche transaction, net of issuance costs	245,387		245,387
Issuance of common stock from Roche transaction, net of issuance costs, Shares		5,000,000
Stock-based compensation expense	11,176		11,176
Other comprehensive loss	(178)			$ (178)
Net loss	(89,631)				(89,631)
Ending Balance at Dec. 31, 2015	257,689	$ 3	489,480	(178)	(231,616)
Ending Balance, shares at Dec. 31, 2015		34,513,845
Exercise of stock options	652		652
Exercise of stock options, shares		167,853
Vesting of restricted stock	9	$ 1	8
Vesting of restricted stock, shares		599,303
Stock-based compensation expense	19,524		19,524
Other comprehensive loss	164			164
Net loss	(113,192)				(113,192)
Ending Balance at Dec. 31, 2016	164,846	$ 4	509,664	(14)	(344,808)
Ending Balance, shares at Dec. 31, 2016		35,281,001
Cumulative effective adjustment for ASU 2016-09 (Note 2)			75		(75)
Exercise of stock options	$ 5,963		5,963
Exercise of stock options, shares	549,280	540,695
Vesting of restricted stock, shares		720,074
Stock-based compensation expense	$ 22,202		22,202
Other comprehensive loss	123			123
Net loss	(161,466)				(161,466)
Ending Balance at Dec. 31, 2017	$ 31,668	$ 4	$ 537,904	$ 109	$ (506,349)
Ending Balance, shares at Dec. 31, 2017		36,541,770