Commitments and Contingencies - Research Collaboration and Licensing Agreements (Details) (USD $)	1 Months Ended	12 Months Ended
	Jul. 31, 2008	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Agreements with Royalty Provisions
Research Collaboration and Licensing Arrangements [Line Items]
Royalty rate, minimum		1.00%
Royalty rate, maximum		16.50%
Royalty expense		$ 59,500,000	$ 3,200,000	$ 1,700,000
License agreement with Genentech
Research Collaboration and Licensing Arrangements [Line Items]
Amount of payment to be made upon reaching minimum sales target		60,000,000
Cumulative sales range, minimum		400,000,000
Cumulative sales range, maximum		3,000,000,000
Royalty payment percentage of cumulative sales within cumulative sales range		4.75%
Royalty payment percentage of cumulative sales over range		5.50%
Cellectis Agreement
Research Collaboration and Licensing Arrangements [Line Items]
Non-refundable advance payment made under license agreement	12,500,000
Amortization of advance payment		1,000,000	1,000,000	900,000
Unamortized balance of advance payment		4,700,000	5,700,000
Future amortization expense of advance payment [Abstract]
2013		1,000,000
2014		900,000
2015		900,000
2016		900,000
2017		$ 900,000