Stockholders' Equity (Details) (USD $)	1 Months Ended		3 Months Ended	134 Months Ended		0 Months Ended		1 Months Ended		0 Months Ended	1 Months Ended
	Mar. 31, 2012	Mar. 31, 2010	Mar. 31, 2012	Mar. 31, 2013 item	Dec. 31, 2012	Jul. 26, 2012 GSK SPA	Jul. 17, 2012 GSK SPA	Jul. 31, 2012 GSK SPA	Mar. 31, 2013 GSK SPA	Oct. 28, 2010 GSK Original Collaboration Agreement	Oct. 31, 2010 GSK Original Collaboration Agreement
Stockholders' Equity
Authorized number of shares of common stock				125,000,000	125,000,000
Voting right for each share held, number				1
Collaborative Agreements
Stock issued from collaboration agreement (in shares)		4,900,000					2,900,000	2,900,000		6,900,000	6,870,000
Common stock issued (in shares)	11,500,000	4,950,000
Offering price (in dollars per share)	$ 5.70	$ 3.74	$ 5.70				$ 6.30	$ 6.30		$ 4.56	$ 4.56
Total purchase price (in dollars)						$ 18,600,000	$ 18,600,000
Aggregate offering proceeds from issuance of common stock (in dollars)		18,500,000	65,600,000
Ownership position in the company (as a percent)									19.80%
Warrants issued (in shares)		1,900,000
Number of common stock sold in a unit (in shares)		1
Number of warrants sold in a unit (in shares)		1
Portion of share purchased against a unit		0.375
Warrant term		4 years
Exercise period of warrants on or after date of issue		6 months
Exercise price of warrants (in dollars per share)		$ 4.43
Proceeds from the issuance of common stock and warrants (in dollars)	$ 62,000,000	$ 17,100,000	$ 62,063,000	$ 193,441,000						$ 31,000,000	$ 31,000,000
Warrants outstanding				1,400,000