CONDENSED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Cash Flows from Operating Activities:
Net Loss	$ (30,174)	$ (29,903)
Adjustments to Reconcile Net Loss to Net Cash Used in Operating Activities:
Depreciation and Amortization	672	579
Amortization of Intangible Assets	253	253
Amortization and Premium of Marketable Securities	(1,219)	(2,129)
Stock-Based Compensation Expense	2,281	1,250
Non-Cash Expense	72
Changes in Operating Assets and Liabilities:
Accounts and Other Receivables	(252)	184
Prepaid and Other Current Assets	(38)	(1,051)
Other Assets	(166)	16
Accounts Payable and Accrued Expenses	(3,220)	(1,669)
Other Liabilities	(798)	595
Net Cash Used in Operating Activities	(32,589)	(31,875)
Cash Flows from Investing Activities:
Sales and Maturities of Marketable Securities	57,067	31,283
Purchases of Marketable Securities	(120,541)	(112,416)
Acquisition of Property and Equipment	(1,561)	(195)
Net Cash Used in Investing Activities	(65,035)	(81,328)
Cash Flows from Financing Activities:
Net Proceeds from Stock Issuances	188,774
Proceeds from Issuance of Stock from Employee Benefit Plans	2,985	702
Net Cash Provided by Financing Activities	191,759	702
Effect of Exchange Rate Changes on Cash and Cash Equivalents	15	1
Net Increase (Decrease) in Cash and Cash Equivalents	94,150	(112,500)
Cash and Cash Equivalents at Beginning of Period	28,020	169,402
Cash and Cash Equivalents at End of Period	122,170	56,902
Non-cash Investing Activities
Acquisition of Property and Equipment included in Accounts Payable and Accrued Expenses	$ 839