Income Tax - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Disclosure [Line Items]
Increase (Decrease) in valuation allowance									$ (800,000)	$ 2,000,000	$ 7,200,000
Federal statutory income tax rate									34.00%
Undistributed earnings, federal statutory income taxes									(5,329,000)	(5,725,000)	(5,718,000)
Net loss	(1,716,000)	(3,460,000)	(1,907,000)	(8,834,000)	(7,735,000)	2,260,000	(6,409,000)	(5,098,000)	(15,917,000)	(16,982,000)	(16,911,000)
Research and development tax credit carryforwards	30,800,000				31,400,000				30,800,000	31,400,000
Internal Revenue Service (IRS)
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	360,200,000								360,200,000
Research and development tax credit carryforwards	20,500,000								20,500,000
Internal Revenue Service (IRS) | Minimum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year									2018
Internal Revenue Service (IRS) | Maximum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year									2032
State and Local Jurisdiction
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	293,200,000								293,200,000
Research and development tax credit carryforwards	15,600,000								15,600,000
State and Local Jurisdiction | Minimum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year									2013
State and Local Jurisdiction | Maximum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year									2032
Cerus Europe B.V.
Income Tax Disclosure [Line Items]
Undistributed earning of foreign subsidiary									2,100,000
Undistributed earnings, federal statutory income taxes									$ 700,000