STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flows From Operating Activities:
Net Loss	$ (128,530)	$ (127,197)	$ (118,080)
Adjustments to Reconcile Net Loss to Net Cash Used in Operating Activities:
Depreciation and Amortization	3,095	2,998	2,388
Amortization of Intangible Assets	997	1,013	1,013
Amortization and Premium of Marketable Securities, Net	926	350	95
Realized Gain on Sales and Maturities of Marketable Securities			(11)
Loss on Sale or Disposal of Assets	81		6
Stock-Based Compensation Expense	15,317	12,774	6,852
Non-Cash Expense	1,638	288	72
Changes in Operating Assets and Liabilities:
Accounts and Other Receivables	(814)	(543)	62
Prepaid and Other Current Assets	1,320	(653)	(2,026)
Other Assets	(89)	6	65
Accounts Payable and Accrued Expenses	(4,970)	4,875	1,450
Other Liabilities	(2,007)	7,202	6,577
Net Cash Used in Operating Activities	(113,036)	(98,887)	(101,537)
Cash Flows From Investing Activities:
Sales and Maturities of Marketable Securities	242,792	161,090	109,232
Purchases of Marketable Securities	(173,925)	(206,405)	(148,314)
Investment in Other	(1,801)
Cash Acquired in Kolltan Acquisition, net	4,592
Acquisition of Property and Equipment	(2,751)	(4,876)	(1,929)
Net Cash Provided by (Used in) Investing Activities	68,907	(50,191)	(41,011)
Cash Flows From Financing Activities:
Net Proceeds from Stock Issuances	13,946	188,840
Proceeds from Issuance of Stock from Employee Benefit Plans	536	4,311	1,171
Net Cash Provided by Financing Activities	14,482	193,151	1,171
Effect of Exchange Rate Changes on Cash and Cash Equivalents		15	(5)
Net (Decrease) Increase in Cash and Cash Equivalents	(29,647)	44,088	(141,382)
Cash and Cash Equivalents at Beginning of Period	72,108	28,020	169,402
Cash and Cash Equivalents at End of Period	42,461	72,108	28,020
Non-cash Investing Activities
Accrued construction in progress	159	$ 75	$ 1,027
Non-cash Supplemental Disclosure
Shares issued to former Kolltan executive for settlement of severance	426
Shares issued in connection with Kolltan Acquisition	$ 73,397