CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Cash Flows from Operating Activities:
Net Loss	$ (62,533)	$ (58,177)
Adjustments to Reconcile Net Loss to Net Cash Used in Operating Activities:
Depreciation and Amortization	1,384	1,161
Amortization of Intangible Assets	507	507
Amortization and Premium of Marketable Securities	(1,228)	(1,436)
Stock-Based Compensation Expense	4,808	2,761
Non-Cash Expense	144
Changes in Operating Assets and Liabilities:
Accounts and Other Receivables	(7)	(36)
Prepaid and Other Current Assets	(1,837)	(1,825)
Other Assets	(98)	32
Accounts Payable and Accrued Expenses	2,524	(1,033)
Other Liabilities	(1,015)	5,315
Net Cash Used in Operating Activities	(57,351)	(52,731)
Cash Flows from Investing Activities:
Sales and Maturities of Marketable Securities	86,345	50,587
Purchases of Marketable Securities	(164,434)	(126,222)
Acquisition of Property and Equipment	(3,108)	(1,094)
Net Cash Used in Investing Activities	(81,197)	(76,729)
Cash Flows from Financing Activities:
Net Proceeds from Stock Issuances	188,840
Proceeds from Issuance of Stock from Employee Benefit Plans	3,257	856
Net Cash Provided by Financing Activities	192,097	856
Effect of Exchange Rate Changes on Cash and Cash Equivalents	15	(1)
Net Increase (Decrease) in Cash and Cash Equivalents	53,564	(128,605)
Cash and Cash Equivalents at Beginning of Period	28,020	169,402
Cash and Cash Equivalents at End of Period	81,584	$ 40,797
Non-cash Investing Activities
Acquisition of Property and Equipment included in Accounts Payable and Accrued Expenses	$ 503