CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Cash Flows from Operating Activities:
Net Loss	$ (34,673)	$ (30,174)
Adjustments to Reconcile Net Loss to Net Cash Used in Operating Activities:
Depreciation and Amortization	742	672
Amortization of Intangible Assets	253	253
Amortization and Premium of Marketable Securities, Net	529	(1,219)
Loss on Sale or Disposal of Assets	70
Stock-Based Compensation Expense	3,940	2,281
Non-Cash Expense	1,638	72
Changes in Operating Assets and Liabilities:
Accounts and Other Receivables	123	(252)
Prepaid and Other Current Assets	(1,741)	(38)
Other Assets		(166)
Accounts Payable and Accrued Expenses	(4,957)	(3,220)
Other Liabilities	(826)	(798)
Net Cash Used in Operating Activities	(34,902)	(32,589)
Cash Flows from Investing Activities:
Sales and Maturities of Marketable Securities	74,939	57,067
Purchases of Marketable Securities	(48,860)	(120,541)
Acquisition of Property and Equipment	(540)	(1,561)
Net Cash Provided by (Used in) Investing Activities	25,539	(65,035)
Cash Flows from Financing Activities:
Net Proceeds from Stock Issuances		188,774
Proceeds from Issuance of Stock from Employee Benefit Plans	280	2,985
Net Cash Provided by Financing Activities	280	191,759
Effect of Exchange Rate Changes on Cash and Cash Equivalents		15
Net (Decrease) Increase in Cash and Cash Equivalents	(9,083)	94,150
Cash and Cash Equivalents at Beginning of Period	72,108	28,020
Cash and Cash Equivalents at End of Period	63,025	122,170
Non-cash Investing Activities
Acquisition of Property and Equipment included in Accounts Payable and Accrued Expenses	$ 278	$ 839