Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Common Stock [Member] Pregenen [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Pregenen [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Series A -2 Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Series D Convertible Preferred Stock [Member]	Series A -1 Convertible Preferred Stock [Member]
Beginning balance at Dec. 31, 2012	$ (55,747)	$ 3		$ 15,267			$ (73,354)	$ 7,137	$ 40,321	$ 12,382	$ 60,000	$ 2,337
Beginning balance, shares at Dec. 31, 2012		309						22,304	115,204	39,943	120,409	12,981
Vesting of restricted stock issued in exchange for nonrecourse note (in shares)		41
Vesting of restricted stock units, shares		45
Proceeds from issuance of common stock upon public offering, net of issuance costs	104,921	$ 69		104,852
Proceeds from issuance of common stock upon public offering, net of issuance costs, shares		6,832
Conversion of convertible preferred stock into common stock	119,841	$ 164		122,014				$ (7,137)	$ (40,321)	$ (12,382)	$ (60,000)	$ (2,337)
Conversion of convertible preferred stock into common stock, shares		16,389						(22,304)	(115,204)	(39,943)	(120,409)	(12,981)
Reclassification of warrants to purchase preferred stock to stockholders' equity	655			655
Repayment of nonrecourse note	344			344
Exercise of common stock warrants		$ 1		(1)
Exercise of common stock warrants, shares		102
Exercise of stock options	483	$ 2		481
Exercise of stock options, shares		222
Stock-based compensation	6,491			6,491
Net loss	(25,321)						(25,321)
Ending balance at Dec. 31, 2013	151,667	$ 239		250,103			(98,675)
Ending balance, shares at Dec. 31, 2013		23,940
Vesting of restricted stock issued in exchange for nonrecourse note		$ 1		(1)
Vesting of restricted stock issued in exchange for nonrecourse note (in shares)		69
Proceeds from issuance of common stock upon public offering, net of issuance costs	353,042	$ 65		352,977
Proceeds from issuance of common stock upon public offering, net of issuance costs, shares		6,498
Issuance of common stock in connection with acquisition	19,474	$ 4		19,470
Issuance of common stock in connection with acquisition, shares		411
Exercise of common stock warrants		$ 1		(1)
Exercise of common stock warrants, shares		114
Exercise of stock options	5,049	$ 13		5,036
Exercise of stock options, shares		1,306
Issuance of common stock in exchange for consulting services to non-employees	42			42
Issuance of common stock in exchange for consulting services, shares		2
Stock-based compensation	10,763			10,763
Unrealized loss on available-for-sale securities, net of tax	(71)					$ (71)
Net loss	(48,709)						(48,709)
Ending balance at Dec. 31, 2014	$ 491,257	$ 323		638,389		(71)	(147,384)
Ending balance, shares at Dec. 31, 2014	32,340	32,340
Vesting of restricted stock units		$ 1		(1)
Vesting of restricted stock units, shares		62
Proceeds from issuance of common stock upon public offering, net of issuance costs	$ 477,247	$ 29		477,218
Proceeds from issuance of common stock upon public offering, net of issuance costs, shares		2,941
Issuance of common stock in connection with acquisition			$ 1		$ (1)
Issuance of common stock in connection with acquisition, shares			94
Exercise of common stock warrants		$ 2		(2)
Exercise of common stock warrants, shares		164
Exercise of stock options	$ 9,383	$ 13		9,370
Exercise of stock options, shares	1,283	1,282
Purchase of common stock under ESPP	$ 492			492
Purchase of common stock under ESPP, shares		11
Stock-based compensation	41,120			41,120
Unrealized loss on available-for-sale securities, net of tax	(2,220)					(2,220)
Net loss	(166,783)						(166,783)
Ending balance at Dec. 31, 2015	$ 850,496	$ 369		$ 1,166,585		$ (2,291)	$ (314,167)
Ending balance, shares at Dec. 31, 2015	36,894	36,894