STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash Flows from Operating Activities:
Net Loss	$ (127,197)	$ (118,080)	$ (81,550)
Adjustments to Reconcile Net Loss to Net Cash Used in Operating Activities:
Depreciation and Amortization	2,998	2,388	1,932
Amortization of Intangible Assets	1,013	1,013	1,013
Amortization and Premium of Marketable Securities, Net	350	95	(1,366)
Realized Gain on Sales and Maturities of Marketable Securities		(11)
Gain on Sale or Disposal of Assets		6	(21)
Stock-Based Compensation Expense	12,774	6,852	4,548
Non-Cash Expense	288	72	97
Changes in Operating Assets and Liabilities:
Accounts and Other Receivables	(543)	62	(445)
Prepaid and Other Current Assets	(653)	(2,026)	(706)
Other Assets	6	65	276
Accounts Payable and Accrued Expenses	4,875	1,450	7,236
Other Liabilities	7,202	6,577	1,317
Net Cash Used in Operating Activities	(98,887)	(101,537)	(67,669)
Cash Flows from Investing Activities:
Sales and Maturities of Marketable Securities	161,090	109,232	38,894
Purchases of Marketable Securities	(206,405)	(148,314)	(112,118)
Acquisition of Property and Equipment	(4,876)	(1,929)	(4,219)
Proceeds from Sale or Disposal of Assets			21
Net Cash Used in Investing Activities	(50,191)	(41,011)	(77,422)
Cash Flows from Financing Activities:
Net Proceeds from Stock Issuances	188,840		295,697
Proceeds from Issuance of Stock from Employee Benefit Plans	4,311	1,171	5,403
Payments of Term Loan			(11,029)
Payments of Other Liabilities			(472)
Net Cash Provided by Financing Activities	193,151	1,171	289,599
Effect of Exchange Rate Changes on Cash and Cash Equivalents	15	(5)	(3)
Net Increase (Decrease) in Cash and Cash Equivalents	44,088	(141,382)	144,505
Cash and Cash Equivalents at Beginning of Period	28,020	169,402	24,897
Cash and Cash Equivalents at End of Period	72,108	28,020	169,402
Supplemental Disclosure of Cash Flow Information
Cash Paid for Interest			$ 1,209
Non-cash Investing Activities
Accrued construction in progress	$ 75	$ 1,027