CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (11,842)	$ (12,075)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation and amortization	178	175
Gain on sale of assets, net	(794)	(795)
Deferred revenues	0	(730)
Deferred rent	6	38
Amortization of premiums on investments	617	639
Non-cash share-based compensation expense	2,447	1,732
Change in operating assets and liabilities:
Receivables under collaboration agreements and other assets	850	14,090
Accounts payable and accrued liabilities	(1,127)	(968)
Cease-use liability	(102)	(194)
Net cash (used in) provided by operating activities	(9,767)	1,912
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of investments	(108,907)	(66,501)
Sales and maturities of investments	43,391	31,470
Proceeds from sales of property and equipment	40	13
Purchases of property and equipment	(255)	(59)
Net cash used in investing activities	(65,731)	(35,077)
CASH FLOWS FROM FINANCING ACTIVITIES
Issuance of common stock	136,241	966
Net cash provided by financing activities	136,241	966
Net increase (decrease) in cash and cash equivalents	60,743	(32,199)
Cash and cash equivalents at beginning of the period	44,789	63,754
Cash and cash equivalents at end of the period	$ 105,532	$ 31,555