Consolidated Statements of Stockholders' Equity - 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]
Beginning balance at Dec. 31, 2013	$ 151,667	$ 239		$ 250,103			$ (98,675)
Beginning 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
Issuance of common stock upon public offering, net of issuance costs	353,042	$ 65		352,977
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 gain (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
Vesting of restricted stock units		$ 1		(1)
Vesting of restricted stock units, shares		62
Issuance of common stock upon public offering, net of issuance costs	477,247	$ 29		477,218
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		1,282
Purchase of common stock under ESPP	492			492
Purchase of common stock under ESPP, shares		11
Stock-based compensation	41,120			41,120
Unrealized gain (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
Vesting of restricted stock units		$ 1		(1)
Vesting of restricted stock units, shares		113
Issuance of common stock upon public offering, net of issuance costs	$ 234,731	$ 33		234,698
Issuance of common stock upon public offering, net of issuance costs, shares		3,289
Exercise of stock options	$ 6,145	$ 4		6,141
Exercise of stock options, shares	377	377
Purchase of common stock under ESPP	$ 677			677
Purchase of common stock under ESPP, shares		18
Stock-based compensation	39,756			39,756
Unrealized gain (loss) on available-for-sale securities, net of tax	1,142					1,142
Net loss	(263,507)						(263,507)
Ending balance at Dec. 31, 2016	$ 869,440	$ 407		$ 1,447,856		$ (1,149)	$ (577,674)
Ending balance, shares at Dec. 31, 2016	40,691	40,691