Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Line Items]
Increase (Decrease) in valuation allowance	$ 14,100,000	$ 8,500,000	$ (800,000)
Pretax net losses	(38,560,000)	(43,119,000)	(15,675,000)
Operating loss carryforwards related to windfall tax deductions	2,200,000
Research and development tax credit carryforwards	33,800,000	32,100,000
Internal Revenue Service (IRS)
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	416,000,000
Research and development tax credit carryforwards	22,500,000
Internal Revenue Service (IRS) | Minimum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year	2015
Research and development tax credit carryforwards, expiration year	2018
Internal Revenue Service (IRS) | Maximum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year	2034
Research and development tax credit carryforwards, expiration year	2034
State and Local Jurisdiction
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	255,000,000
Research and development tax credit carryforwards	17,100,000
State and Local Jurisdiction | Minimum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year	2015
State and Local Jurisdiction | Maximum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year	2034
Cerus Europe B.V.
Income Tax Disclosure [Line Items]
Undistributed earning of foreign subsidiary	4,600,000
Unrecognized deferred tax liability for unrepatriated earnings	$ 1,700,000