INCOME TAXES - SUMMARY (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income tax benefit (provision):
Federal									$ 46,598	$ 43,536	$ 30,962
State									10,642	7,328	4,920
Expiration of Net Operating Losses and Research & Development Tax Credits									(155)	(2,302)	(126)
Income tax benefit (provision) before valuation allowance									57,085	48,562	35,756
Deferred tax valuation allowance									$ (57,085)	(48,562)	(35,756)
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	$ (32,684)	$ (31,980)	$ (32,359)	$ (30,174)	$ (31,821)	$ (28,082)	$ (28,274)	$ (29,903)	$ (127,197)	(118,080)	(81,550)
Loss at Statutory Rates									(43,247)	(40,147)	(27,727)
Research and Development Credits									(4,935)	(4,126)	(3,261)
State Taxes									(10,642)	(7,328)	(4,920)
Other									1,584	737	26
Expiration of Net Operating Losses and Research & Development Tax Credits									155	2,302	126
Change in Valuation Allowance									57,085	48,562	$ 35,756
Gross Deferred Tax Assets
Net Operating Loss Carryforwards	180,777				95,577				180,777	95,577
Tax Credit Carryforwards	35,895				29,709				35,895	29,709
Deferred Expenses	48,608				85,889				48,608	85,889
Stock-based Compensation	8,393				5,556				8,393	5,556
Fixed Assets	2,017				1,882				2,017	1,882
Deferred Revenue	1,703				2,078				1,703	2,078
Accrued Expenses and Other	219				202				219	202
Gross Deferred Tax Assets	277,612				220,893				277,612	220,893
Gross Deferred Tax Liabilities
Other Acquired Intangibles	(3,382)				(3,749)				(3,382)	(3,749)
IPR&D Intangibles	(4,661)				(4,661)				(4,661)	(4,661)
Gross Deferred Tax Liabilities	(8,043)				(8,410)				(8,043)	(8,410)
Total Deferred Tax Assets and Liabilities	269,569				212,483				269,569	212,483
Deferred Tax Assets Valuation Allowance	(274,230)				(217,144)				(274,230)	(217,144)
Net Deferred Tax Asset (Liability)	$ (4,661)				$ (4,661)				$ (4,661)	$ (4,661)