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	$ (29,903)	$ (17,332)
Adjustments to Reconcile Net Loss to Net Cash Used in Operating Activities:
Depreciation and Amortization	579	468
Amortization of Intangible Assets	253	253
Amortization and Premium of Marketable Securities	(2,129)	(969)
Stock-Based Compensation Expense	1,250	708
Non-Cash Interest Expense		56
Changes in Operating Assets and Liabilities:
Accounts and Other Receivables	184	(60)
Prepaid and Other Current Assets	(1,051)	(1,234)
Other Assets	16	188
Accounts Payable and Accrued Expenses	(1,669)	206
Other Liabilities	595	516
Net Cash Used in Operating Activities	(31,875)	(17,200)
Cash Flows from Investing Activities:
Sales and Maturities of Marketable Securities	31,283	7,394
Purchases of Marketable Securities	(112,416)	(53,809)
Acquisition of Property and Equipment	(195)	(162)
Net Cash Used in Investing Activities	(81,328)	(46,577)
Cash Flows from Financing Activities:
Net Proceeds from Stock Issuances		114,187
Proceeds from Issuance of Stock from Employee Benefit Plans	702	1,660
Payments of Term Loan		(1,323)
Payments of Other Liabilities		(15)
Net Cash Provided by Financing Activities	702	114,509
Effect of Exchange Rate Changes on Cash and Cash Equivalents	1	(2)
Net (Decrease) Increase in Cash and Cash Equivalents	(112,500)	50,730
Cash and Cash Equivalents at Beginning of Period	169,402	24,897
Cash and Cash Equivalents at End of Period	$ 56,902	$ 75,627