INCOME TAXES (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income tax benefit (provision):
Federal									$ 43,536	$ 30,962	$ 19,394
State									7,328	4,920	4,237
Expiration of Net Operating Losses and Research & Development Tax Credits									(2,302)	(126)	(1,426)
Income tax benefit (provision) before valuation allowance									48,562	35,756	22,205
Deferred tax valuation allowance									(48,562)	(35,756)	(22,205)
Reconciliation between the amount of reported income tax and the amount computed using U.S. Statutory rate
U.S. Statutory rate (as a percent)									34.00%
Pre-tax loss	(31,821)	(28,082)	(28,274)	(29,903)	(22,062)	(23,140)	(19,016)	(17,332)	(118,080)	(81,550)	(59,123)
Loss at Statutory Rates									(40,147)	(27,727)	(20,102)
Research and Development Credits									(4,126)	(3,261)
State Taxes									(7,328)	(4,920)	(4,237)
Other									737	26	708
Expiration of Net Operating Losses and Research & Development Tax Credits									2,302	126	1,426
Change in Valuation Allowance									48,562	35,756	22,205
Gross Deferred Tax Assets
Net Operating Loss Carryforwards	95,577				79,558				95,577	79,558
Tax Credit Carryforwards	29,709				25,582				29,709	25,582
Deferred Expenses	85,889				61,370				85,889	61,370
Stock-based Compensation	5,556				3,926				5,556	3,926
Fixed Assets	1,882				1,638				1,882	1,638
Deferred Revenue	2,078				447				2,078	447
Accrued Expenses and Other	202				176				202	176
Gross Deferred Tax Assets	220,893				172,697				220,893	172,697
Gross Deferred Tax Liabilities
Other Acquired Intangibles	(3,749)				(4,115)				(3,749)	(4,115)
IPR&D Intangibles	(4,661)				(4,661)				(4,661)	(4,661)
Gross Deferred Tax Liabilities	(8,410)				(8,776)				(8,410)	(8,776)
Total Deferred Tax Assets and Liabilities	212,483				163,921				212,483	163,921
Deferred Tax Assets Valuation Allowance	(217,144)				(168,582)				(217,144)	(168,582)
Net Deferred Tax Asset (Liability)	$ (4,661)				$ (4,661)				$ (4,661)	$ (4,661)