Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	6 Months Ended		38 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012
Operating activities
Net loss	$ (34,656)	$ (26,827)	$ (145,155)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	176	86	450
Share-based compensation expense	2,114	397	3,511
Amortization of premiums and discounts on available for sale securities	10	97	471
Gain on sale of available for sale securities		(16)	(34)
Non-cash acquired in-process research and development		7,000	7,000
Changes in operating assets and liabilities:
Prepaid and accrued research and development expenses	1,922	444	5,973
Other operating assets	531	(484)	331
Accounts payable	(1,655)	1,020	1,401
Other accrued expenses	(888)	368	1,812
Net cash used in operating activities	(32,446)	(17,915)	(124,240)
Investing activities
Purchases of property and equipment	(908)	(151)	(2,394)
Purchases of available for sale securities			(27,008)
Maturities and sales of available for sale securities	2,000	6,414	26,571
Net cash provided by (used in) investing activities	1,092	6,263	(2,831)
Financing activities
Proceeds from sale of convertible preferred stock, net of issuance costs		(790)	75,499
Proceeds from sale of common stock, net of issuance costs	69,976		199,335
Proceeds from stock option exercises	42	893	1,193
Proceeds from issuance of convertible promissory notes, net of issuance costs		27,903	27,902
Net cash provided by financing activities	70,018	28,006	303,929
Effect of exchange rate changes on cash and cash equivalents	3	8	45
Increase in cash and cash equivalents	38,667	16,362	176,903
Cash and cash equivalents at beginning of period	138,236	10,508
Cash and cash equivalents at end of period	176,903	26,870	176,903
Non-cash items:
Conversion of convertible preferred stock to common stock			75,499
Conversion of convertible promissory notes and accrued interest to common stock			35,851
Assets recorded for which payment (has)/has not yet occurred	$ (684)