Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Line Items]
Increase in valuation allowance	$ 11,800,000	$ 14,100,000	$ 8,500,000
Loss before income taxes	(59,539,000)	(38,560,000)	$ (43,119,000)
Operating loss carryforwards related to windfall tax deductions	2,300,000
Research and development tax credit carryforwards	35,131,000	33,800,000
Unrecognized tax benefits	0	$ 0
Internal Revenue Service (IRS)
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	466,000,000
Research and development tax credit carryforwards	$ 23,300,000
Internal Revenue Service (IRS) | Minimum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year	2016
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	2035
Research and development tax credit carryforwards, expiration year	2035
State and Local Jurisdiction
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 114,000,000
Research and development tax credit carryforwards	$ 18,000,000
State and Local Jurisdiction | Minimum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year	2016
State and Local Jurisdiction | Maximum
Income Tax Disclosure [Line Items]
Net operating loss carryforwards, expiration year	2035
Cerus Europe B.V.
Income Tax Disclosure [Line Items]
Undistributed earning of foreign subsidiary	$ 4,800,000
Unrecognized deferred tax liability for unrepatriated earnings	$ 1,600,000
Latest Tax Year
Income Tax Disclosure [Line Items]
Tax year remain subject to examination by the taxing jurisdictions	2015