CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flows from Operating Activities:
Net loss	$ (21,003)	$ (83,313)	$ (98,600)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization of property and equipment	7,593	7,594	7,637
Share-based compensation expense	18,322	11,440	13,594
Amortization of premium on investments	595	1,383	2,162
Amortization of intangibles	1,529	1,061	1,061
Changes in operating assets and liabilities:
Collaboration receivable	(49,057)	(10,849)	6,172
Prepaid expenses and other current assets	(1,128)	(14)	(64)
Restricted cash	(1,101)	59	0
Other long-term assets	(1,692)	(92)	0
Accounts payable	(1,032)	(3,380)	1,126
Accrued expenses	2,043	14,151	(1,099)
Collaboration advance	32,895	0	0
Deferred revenue	16,649	(9,015)	3,282
Other current liabilities	(449)	(58)	22
Other long-term liabilities	3,724	(482)	(461)
Net cash provided by (used in) operating activities	7,888	(71,515)	(65,168)
Cash Flows from Investing Activities:
Purchases of property and equipment	(5,609)	(4,068)	(8,360)
Purchases of marketable securities	(360,008)	(405,673)	(111,809)
Proceeds from maturities of marketable securities	445,665	245,907	195,342
Net cash provided by (used in) investing activities	80,048	(163,834)	75,173
Cash Flows from Financing Activities:
Proceeds from public offering of common stock, net of issuance costs	0	148,439	0
Net proceeds from issuance of common stock under ATM facility	0	64,503	18,305
Proceeds from issuance of common stock under stock plans	2,407	24,567	3,273
Repurchase of common stock pursuant to share surrender	(1,066)	(2,048)	0
Net cash provided by financing activities	1,341	235,461	21,578
Increase in cash and cash equivalents	89,277	112	31,583
Cash and cash equivalents, beginning of period	61,461	61,349	29,766
Cash and cash equivalents, end of period	150,738	61,461	61,349
Non-Cash Investing Activity:
Common shares issued to Parivid to settle milestone payment	3,190	0	0
Purchases of property and equipment included in accounts payable and accrued expenses	$ 935	$ 0	$ 0