Stockholders Equity (Details) (USD $)	1 Months Ended		12 Months Ended				1 Months Ended		1 Months Ended					12 Months Ended
	Oct. 31, 2011	Oct. 31, 2010	Dec. 31, 2011 PerShare	Dec. 31, 2010 PerShare	Dec. 31, 2009	Oct. 31, 2011 Warrant Transactions [Member]	Oct. 31, 2011 Call Option [Member]	Oct. 31, 2011 Convertible Senior Notes [Member]	Oct. 31, 2010 Sanofi [Member]	Nov. 30, 2009 Sanofi [Member] Percent	Dec. 31, 2007 Sanofi [Member]	Jan. 31, 2003 Sanofi [Member]	Dec. 31, 2011 Sanofi [Member] Percent	Dec. 31, 2011 Class A Stock [Member] PerShare	Dec. 31, 2010 Class A Stock [Member] PerShare
Class of Stock [Line Items]
Common Stock, shares authorized for issuance (in shares)			160,000,000	160,000,000										40,000,000	40,000,000
Common Stock, par value (in dollars per share)			0.001	0.001										0.001	0.001
Common Stock, conversion features														share-for-share basis
Common Stock, voting rights			one vote per share											ten votes per share
Preferred stock, authorized (in shares)			30,000,000	30,000,000
Number of newly issued, unregistered shares of Common Stock purchased (in shares)									1,017,401		12,000,000	2,799,552
Cash price of newly issued, unregistered shares of Common Stock purchased											$ 312,000,000	$ 45,000,000
Description of period of 'standstill' provisions										Until the later of the fifth anniversaries of the expiration or earlier termination of the License and Collaboration Agreement
Maximum percentage of outstanding shares that may be acquired, under 'standstill' provisions (in hundredths)										25.00%			30.00%
Percentage of outstanding shares of Common Stock related to prohibition on selling to beneficial owners (in hundredths)										5.00%
Percentage of outstanding shares or voting rights under the amended investor agreement (in hundredths)										10.00%
Issuance of Common Stock in a public offering (in shares)		6,325,000
Net proceeds from issuance of Common Stock				174,828,000
Debt Instrument [Line Items]
Principal amount								400,000,000
Equity component of convertible senior notes, net of issuance costs	120,623,000
Option Indexed to Issuer's Equity [Line Items]
Purchase of convertible note hedges							117,500,000
Class of Warrant or Right [Line Items]
Strike price of the warrant transaction (in dollars per share)						$ 103.41
Proceeds received from the Warrant			$ 93,800,000	$ 0	$ 0