CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash Flows from Operating Activities:
Net loss	$ (62,542)	$ (54,149)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	5,726	5,799
Share-based compensation expense	14,756	6,787
Amortization of premium on investments	524	987
Amortization of intangibles	1,240	796
Changes in operating assets and liabilities:
Collaboration receivable	(24,567)	(2,693)
Prepaid expenses and other current assets	(1,337)	136
Restricted cash	(1,101)	59
Other long-term assets	(1,121)	0
Accounts payable	5,748	(1,714)
Accrued expenses	(7,960)	3,779
Deferred revenue	33,067	(6,572)
Other current liabilities	(377)	25
Other long-term liabilities	1,789	(422)
Net cash used in operating activities	(36,155)	(47,182)
Cash Flows from Investing Activities:
Purchases of property and equipment	(4,806)	(2,747)
Purchases of marketable securities	(264,905)	(281,500)
Proceeds from maturities of marketable securities	350,531	105,844
Net cash provided by (used in) investing activities	80,820	(178,403)
Cash Flows from Financing Activities:
Proceeds from public offering of common stock, net of issuance costs	0	148,439
Net proceeds from issuance of common stock under ATM facilities	0	64,503
Proceeds from issuance of common stock under stock plans	1,218	23,329
Repurchase of common stock pursuant to share surrender	(1,065)	(2,048)
Net cash provided by financing activities	153	234,223
Increase in cash and cash equivalents	44,818	8,638
Cash and cash equivalents, beginning of period	61,461	61,349
Cash and cash equivalents, end of period	106,279	69,987
Non-Cash Investing Activity:
Common shares issued to Parivid to settle milestone payment	$ 3,190	$ 0