CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash Flows from Operating Activities:
Net Loss	$ (94,513)	$ (86,258)
Adjustments to Reconcile Net Loss to Net Cash Used in Operating Activities:
Depreciation and Amortization	2,151	1,764
Amortization of Intangible Assets	760	760
Amortization and Premium of Marketable Securities, Net	(501)	(754)
Realized Gain on Sales and Maturities of Marketable Securities		(11)
Stock-Based Compensation Expense	8,677	4,719
Non-Cash Expense	216
Changes in Operating Assets and Liabilities:
Accounts and Other Receivables	(100)	(2,416)
Prepaid and Other Current Assets	(478)	(1,428)
Other Assets	6	49
Accounts Payable and Accrued Expenses	(70)	(740)
Other Liabilities	(1,290)	4,992
Net Cash Used in Operating Activities	(85,142)	(79,323)
Cash Flows from Investing Activities:
Sales and Maturities of Marketable Securities	129,349	89,215
Purchases of Marketable Securities	(193,815)	(146,159)
Acquisition of Property and Equipment	(4,469)	(1,766)
Net Cash Used in Investing Activities	(68,935)	(58,710)
Cash Flows from Financing Activities:
Net Proceeds from Stock Issuances	188,840
Proceeds from Issuance of Stock from Employee Benefit Plans	3,993	1,160
Net Cash Provided by Financing Activities	192,833	1,160
Effect of Exchange Rate Changes on Cash and Cash Equivalents	15	(4)
Net Increase (Decrease) in Cash and Cash Equivalents	38,771	(136,877)
Cash and Cash Equivalents at Beginning of Period	28,020	169,402
Cash and Cash Equivalents at End of Period	66,791	$ 32,525
Non-cash Investing Activities
Acquisition of Property and Equipment included in Accounts Payable and Accrued Expenses	$ 38