CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities:
Net loss	$ (88,096)	$ (21,003)	$ (83,313)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation and amortization of property and equipment	8,023	7,593	7,594
Share-based compensation expense	16,127	18,322	11,440
Amortization of premium on investments	167	595	1,383
Amortization of intangibles	1,153	1,529	1,061
Loss on disposal of assets	61	0	0
Changes in operating assets and liabilities:
Collaboration receivable	55,194	(49,057)	(10,849)
Prepaid expenses and other current assets	(2,098)	(1,128)	(14)
Restricted cash	(1,271)	(1,101)	59
Other long-term assets	1,229	(1,692)	(92)
Accounts payable	7,446	(1,032)	(3,380)
Accrued expenses	(6,253)	2,043	14,151
Collaboration liabilities	(23,637)	32,895	0
Deferred revenue	(5,015)	16,649	(9,015)
Lease incentive	4,051	0	0
Other current liabilities	(66)	(449)	(58)
Other long-term liabilities	2,629	3,724	(482)
Net cash (used in) provided by operating activities	(30,356)	7,888	(71,515)
Cash Flows from Investing Activities:
Purchases of property and equipment	(17,127)	(5,609)	(4,068)
Proceeds from disposal of equipment	267	0	0
Purchases of marketable securities	(524,888)	(360,008)	(405,673)
Proceeds from maturities of marketable securities	420,669	445,665	245,907
Net cash (used in) provided by investing activities	(121,079)	80,048	(163,834)
Cash Flows from Financing Activities:
Proceeds from public offering of common stock, net of issuance costs	0	0	148,439
Net proceeds from issuance of common stock under ATM facility	64,090	0	64,503
Proceeds from issuance of common stock under stock plans	10,258	2,407	24,567
Repurchase of common stock pursuant to share surrender	0	(1,066)	(2,048)
Net cash provided by financing activities	74,348	1,341	235,461
Net (decrease) increase in cash and cash equivalents	(77,087)	89,277	112
Cash and cash equivalents, beginning of period	150,738	61,461	61,349
Cash and cash equivalents, end of period	73,651	150,738	61,461
Non-Cash Activity:
Common shares issued to Parivid to settle milestone payment	0	3,190	0
Purchases of property and equipment included in accounts payable and accrued expenses	1,228	935	0
Receivable due from stock option exercises	93	0	0
Impact of adopting ASU 2016-09	$ 783	$ 0	$ 0