Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended		36 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013
Operating activities
Net loss	$ (18,869)	$ (9,329)	$ (106,005)
Adjustments to reconcile net loss to net cash used in operating activities:
Acquired in-process research and development			15,130
Depreciation	37	12	143
Increase in fair value of investor rights obligation			1,661
Share based compensation expense	770	206	2,878
Changes in operating assets and liabilities:
Other assets	294	(215)	(1,422)
Accounts payable	(1,065)	221	2,105
Accrued expenses	956	655	9,501
Other liabilities	23	(3)	26
Net cash used in operating activities	(17,854)	(8,453)	(75,983)
Investing activities
Acquisition of product candidate licenses and milestone payments			(14,500)
Purchase of property and equipment	(308)	(30)	(633)
Net cash used in investing activities	(308)	(30)	(15,133)
Financing activities
Accumulated issuance costs of planned initial public offering		(1,054)
Proceeds from sale of common stock, net of issuance costs	91,312		169,272
Proceeds from exercise of stock options	49		82
Proceeds from sale of convertible preferred stock and related investor rights, net of issuance costs		58,356	120,406
Net cash provided by financing activities	91,361	57,302	289,760
Increase in cash and cash equivalents	73,199	48,819	198,644
Cash and cash equivalents at beginning of period	125,445	39,825
Cash and cash equivalents at end of period	198,644	88,644	198,644
Non-cash investing and financing activities
Issuance of Series O convertible preferred stock			630
Settlement of investor rights obligation			3,829
Conversion of convertible preferred stock to common stock			$ 122,697