Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jul. 31, 2011 South San Francisco California [Member] sqft	Sep. 30, 2011 Headquarters [Member] Redwood City, California [Member] Lease and Sublease [Member] sqft	Sep. 30, 2011 Headquarters [Member] Redwood City, California [Member] Sublease Agreement [Member]	Sep. 30, 2011 Headquarters [Member] Redwood City, California [Member] Lease agreement [Member]	Dec. 31, 2012 Research Laboratories [Member] Redwood City, California [Member] Lease agreement [Member] sqft
Commitments And Contingencies [Line Items]
Square feet of laboratory space and/or office space				28,180	67,905			6,489
Lease agreement expiration date				Apr. 30, 2017		Feb. 28, 2010	Sep. 30, 2011	Aug. 31, 2012
Aggregate rent over life of lease				$ 3,400,000
Security deposit for lease				60,000
Rent expense	700,000	1,300,000	1,200,000
Purchase commitments	$ 4,600,000