Condensed Statements of Cash Flows (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Operating activities
Net loss	$ (26,384,000)	$ (2,374,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of premium on marketable securities	237,000
Amortization of debt issuance cost	170,000	104,000
Depreciation	167,000	252,000
Revaluation of redeemable convertible preferred stock warrants		476,000
Stock-based compensation	2,732,000	809,000
Changes in assets and liabilities:
Prepaid expenses and other current assets	262,000	(3,595,000)
Restricted cash	4,845,000	(11,413,000)
Deferred licensor payment		352,000
Other assets		(6,000)
Accounts payable and other accrued liabilities	(1,404,000)	(1,384,000)
Deferred revenue		(3,847,000)
Deferred development funds	(5,111,000)	12,329,000
Deferred long term payable to licensor		1,268,000
Deferred rent	2,000	1,000
Net cash used in operating activities	(24,484,000)	(7,028,000)
Investing activities
Purchases of marketable securities	(32,386,000)
Investment in property and equipment	(42,000)	(131,000)
Net cash used in investing activities	(32,428,000)	(131,000)
Financing activities
Proceeds from common stock option exercises	641,000	48,000
Borrowings on credit facility	10,000,000
Repayment on credit facility	(454,000)
Net cash provided by financing activities	10,187,000	48,000
Net decrease in cash and cash equivalents	(46,725,000)	(7,111,000)
Cash and cash equivalents at beginning of period	79,311,000	34,577,000
Cash and cash equivalents at end of period	32,586,000	27,466,000
Supplemental disclosures
Issuance of common stock for services rendered	101,000	43,000
Interest expense paid in cash	$ 681,000