Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Redeemable Convertible Preferred Stock [Member]	Series B Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2012	$ (43,397)	$ 42,962	$ 55,696		$ 3,422		$ (46,819)
Beginning Balance, shares at Dec. 31, 2012		43,750,000	24,762,134	2,727,443
Issuance costs related to Series B preferred stock offering			$ (10)
Accretion of redeemable convertible preferred stock to redemption value	(139)	$ 125	14		(139)
Conversion of redeemable convertible preferred stock into common stock	98,787	$ (43,087)	$ (55,700)	$ 2	98,785
Conversion of redeemable convertible preferred stock into common stock, shares		(43,750,000)	(24,762,134)	17,128,024
Reclassification of warrant to purchase redeemable convertible preferred stock into warrant to purchase common stock	1,605				1,605
Issuance of common stock from initial public offering, net of underwriters' discounts and issuance costs	110,381			$ 1	110,380
Issuance of common stock from initial public offering, net of underwriters' discounts and issuance costs, shares				6,772,221
Issuance of common stock in exchange for services	4				4
Issuance of common stock in exchange for services, shares				633
Exercise of warrant for common stock				44,566
Exercise of stock options	49				49
Exercise of stock options, shares				48,863
Vesting of restricted stock	49				49
Vesting of restricted stock, shares				909,031
Stock-based compensation expense	7,316				7,316
Net loss	(42,944)						(42,944)
Ending Balance at Dec. 31, 2013	131,711			$ 3	221,471		(89,763)
Ending Balance, shares at Dec. 31, 2013				27,630,781
Exercise of stock options	637				637
Exercise of stock options, shares				212,822
Vesting of restricted stock	54				54
Vesting of restricted stock, shares				380,355
Stock-based compensation expense	5,989				5,989
Net loss	(52,222)						(52,222)
Ending Balance at Dec. 31, 2014	$ 86,169			$ 3	228,151		(141,985)
Ending Balance, shares at Dec. 31, 2014				28,223,958
Conversion of redeemable convertible preferred stock into common stock, shares	17,128,024
Exercise of stock options	$ 4,739				4,739
Exercise of stock options, shares	1,070,457			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