STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows From Operating Activities:
Net Loss	$ (93,031)	$ (128,530)	$ (127,197)
Adjustments to Reconcile Net Loss to Net Cash Used in Operating Activities:
Depreciation and Amortization	4,414	3,095	2,998
Amortization of Intangible Assets	896	997	1,013
Amortization and Premium of Marketable Securities, Net	(290)	926	350
Loss on Sale or Disposal of Assets	55	81
In-Process Research and Development Impairment	13,000
Gain on Fair Value Remeasurement of Contingent Consideration	(800)
Non-Cash Income Tax Benefit	(24,282)
Stock-Based Compensation Expense	12,312	15,317	12,774
Non-Cash Expense		1,638	288
Changes in Operating Assets and Liabilities:
Accounts and Other Receivables	(96)	(814)	(543)
Prepaid and Other Current Assets	793	1,320	(653)
Other Assets	205	(89)	6
Accounts Payable and Accrued Expenses	(8,744)	(4,970)	4,875
Other Liabilities	(4,363)	(2,007)	7,202
Net Cash Used in Operating Activities	(99,931)	(113,036)	(98,887)
Cash Flows From Investing Activities:
Sales and Maturities of Marketable Securities	219,236	242,792	161,090
Purchases of Marketable Securities	(170,980)	(173,925)	(206,405)
Investment in Other		(1,801)
Cash Acquired in Kolltan Acquisition, Net		4,592
Acquisition of Property and Equipment	(1,788)	(2,751)	(4,876)
Net Cash Provided by (Used in) Investing Activities	46,468	68,907	(50,191)
Cash Flows From Financing Activities:
Net Proceeds from Stock Issuances	51,025	13,946	188,840
Proceeds from Issuance of Stock from Employee Benefit Plans	265	536	4,311
Net Cash Provided by Financing Activities	51,290	14,482	193,151
Effect of Exchange Rate Changes on Cash and Cash Equivalents			15
Net (Decrease) Increase in Cash and Cash Equivalents	(2,173)	(29,647)	44,088
Cash and Cash Equivalents at Beginning of Period	42,461	72,108	28,020
Cash and Cash Equivalents at End of Period	40,288	42,461	72,108
Non-cash Investing Activities
Accrued construction in progress	20	159	$ 75
Non-cash Supplemental Disclosure
Shares issued to former Kolltan executive for settlement of severance	$ 344	426
Shares issued in connection with Kolltan Acquisition		$ 73,397