Summary of Significant Accounting Policies (Details) (USD $) Share data in Thousands, unless otherwise specified	12 Months Ended				12 Months Ended																12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 Computer Equipment [Member]	Dec. 31, 2012 Furniture and Fixtures [Member]	Dec. 31, 2012 Leasehold Improvements [Member]	Dec. 31, 2012 Minimum [Member] Machinery and Equipment [Member]	Dec. 31, 2012 Minimum [Member] Leasehold Improvements [Member]	Dec. 31, 2011 Minimum [Member] Leasehold Improvements [Member]	Dec. 31, 2012 Maximum [Member] Machinery and Equipment [Member]	Dec. 31, 2012 Maximum [Member] Leasehold Improvements [Member]	Dec. 31, 2011 Maximum [Member] Leasehold Improvements [Member]	Dec. 31, 2012 Money Market Funds [Member]	Dec. 31, 2011 Money Market Funds [Member]	Dec. 31, 2012 Commercial Bank [Member]	Dec. 31, 2011 Commercial Bank [Member]	Dec. 31, 2012 Restricted Stock [Member]	Dec. 31, 2011 Restricted Stock [Member]	Dec. 31, 2010 Restricted Stock [Member]	Dec. 31, 2011 Salix Agreement [Member]	Jun. 30, 2011 Salix Agreement [Member] License [Member]	Dec. 31, 2011 Salix Agreement [Member] License [Member]	Jun. 30, 2011 Salix Agreement [Member] Reimbursable Development Services [Member]	Dec. 31, 2012 Salix Agreement [Member] Reimbursable Development Services [Member]	Dec. 31, 2011 Salix Agreement [Member] Reimbursable Development Services [Member]	Dec. 31, 2011 Salix Agreement [Member] Participation in Joint Committees [Member]	Dec. 31, 2008 Ono Agreement [Member]	Dec. 31, 2012 Ono Agreement [Member]	Dec. 31, 2012 MedImmune Agreement [Member]	Dec. 31, 2012 CytoDyn Agreement [Member]
Revenue Recognition [Abstract]
Upfront license fee, received																					$ 60,000,000		$ 58,400,000			$ 1,100,000	$ 500,000	$ 15,000,000			$ 3,500,000
Upfront license fee, receivable																														5,000,000
Deferred Revenue																														3,000	673,000
Additional license fees receivable upon achievement of development milestones																													20,000,000
Collaboration Revenue	8,525,000	76,764,000	1,413,000																			58,400,000		1,100,000	200,000	59,600,000				4,997,000	2,827,000
Shares of unvested restricted stock outstanding (in shares) [Line Items]
Shares of unvested restricted stock outstanding (in shares)	28	98																28	98	341
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	58,838,000	70,105,000	47,918,000	90,903,000										56,224,000	64,068,000	2,614,000	6,037,000
Auction Rate Securities [Abstract]
Auction rate securities	3,240,000	3,332,000
Decrease in temporary impairment of auction rate securities	8,000
Amount of temporary impairment	260,000	268,000
Fair Value Measurements [Abstract]
Other current assets	1,692,000	919,000
Restricted cash	150,000	200,000
Estimated useful lives of fixed assets [Abstract]
Estimated useful life					3 years	5 years		5 years	8 years 6 months	1 year 3 months 18 days	7 years	10 years 9 months 18 days	10 years 9 months 18 days
Useful life description							Earlier of life of improvement or lease
Deferred Lease Liability and Incentive [Abstract]
Deferred lease incentive	115,000	115,000
Total other current liabilities	115,000	115,000
Deferred lease liability	273,000	577,000
Deferred lease incentive	805,000	920,000
Total other liabilities	1,078,000	1,497,000
Impairment of Long-Lived Assets [Abstract]
Impairment loss		$ 22,000
Income Taxes [Abstract]
Percentage of income tax benefit realization (in hundredths)	50.00%