Statements of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating activities
Net loss	$ (36,799,000)	$ (11,152,000)	$ (16,031,000)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Amortization of (discount) on marketable securities			(16,000)
Amortization of debt issuance cost	272,000
Amortization of redeemable convertible preferred stock warrant value		71,000
Depreciation	635,000	54,000	174,000
Loss on disposal of property and equipment		4,000
Revaluation of redeemable convertible preferred stock warrants	422,000	229,000	(570,000)
Stock-based compensation	3,429,000	858,000	1,037,000
Changes in assets and liabilities:
Prepaid expenses and other current assets	561,000	(809,000)	(209,000)
Restricted cash	(8,575,000)	14,416,000	(21,819,000)
Deferred licensor payment	352,000	1,188,000	(1,540,000)
Other assets	(6,000)	(12,000)
Accounts payable and other accrued liabilities	3,203,000	80,000	2,920,000
Deferred revenue	(3,847,000)	(12,984,000)	16,831,000
Deferred development funds	9,223,000	(14,652,000)	19,603,000
Deferred long term payable to licensor	(1,560,000)		1,560,000
Deferred rent	2,000	2,000	5,000
Net cash (used in) provided by operating activities	(32,688,000)	(22,707,000)	1,945,000
Investing activities
Proceeds from sales of marketable securities			12,697,000
Investment in property and equipment	(190,000)	(696,000)	(100,000)
Net cash (used in) provided by investing activities	(190,000)	(696,000)	12,597,000
Financing activities
Borrowings on credit facility	5,000,000
Proceeds from initial public offering, net of issuance costs	72,479,000
Proceeds from common stock option and warrant exercises	133,000	17,000	58,000
Net proceeds from issuance of convertible preferred stock		36,287,000
Net cash provided by financing activities	77,612,000	36,304,000	58,000
Net increase in cash and cash equivalents	44,734,000	12,901,000	14,600,000
Cash and cash equivalents at beginning of period	34,577,000	21,676,000	7,076,000
Cash and cash equivalents at end of period	79,311,000	34,577,000	21,676,000
Supplemental disclosures
Issuance of redeemable convertible preferred stock warrants for senior loan facility commitment	534,000	885,000
Reclassification of warrants to purchase common stock	3,101,000
Conversion of convertible preferred stock to common stock	107,743,000
Issuance of common stock for licensor payment	1,414,000
Issuance of common stock for services rendered	43,000		9,000
Interest expense paid in cash	147,000
Redeemable convertible preferred shares
Supplemental disclosures
Issuance of redeemable convertible preferred stock for licensor payment	158,000		195,000
Issuance of redeemable convertible preferred stock for services rendered			$ 175,000