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Stock-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2018 USD ($) Period $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares	Aug. 31, 2016 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total intrinsic value of options exercised | $		$ 7.1	$ 0.6	$ 1.9
Stock-based compensation, expected dividend yield		0.00%
Weighted average grant-date fair value of stock options granted | $ / shares		$ 2.41	$ 1.98	$ 2.55
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense expected to be recognized | $		$ 10.3
Stock-based compensation, weighted average recognition period		2 years 6 months
Stock-based compensation, expected dividend yield		0.00%	0.00%	0.00%
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense expected to be recognized | $		$ 5.5
Stock-based compensation, weighted average recognition period		1 year 9 months 18 days
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, option to be granted at percentage of fair value of common stock		85.00%
Employee Stock Purchase Plan, offering period		12 months
Number of purchase periods within each offering period | Period		2
Aggregate number of shares of common stock reserved for future issuance		800,000
Stock-based compensation, expected dividend yield		0.00%	0.00%	0.00%
Weighted average grant-date fair value of awards granted | $ / shares		$ 2.29	$ 1.18	$ 1.87
2008 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, option to be granted at percentage of fair value of common stock		100.00%
Aggregate number of shares of common stock reserved for future issuance		22,900,000
Stock-based compensation, award term		10 years
Performance-based stock options, outstanding		20,000
Outstanding options and other stock based awards		17,560,000	17,138,000
Number of shares available for future issuance		3,300,000
2008 Equity Incentive Plan | Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, vesting period		4 years
2008 Equity Incentive Plan | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, vesting period		3 years
Number of Restricted Stock Units Outstanding		2,001,000	1,256,000
Total fair value | $		$ 2.8	$ 1.0	$ 0.0
Weighted average grant-date fair value of awards granted | $ / shares	[1]	$ 4.51
2008 Equity Incentive Plan | Performance-based Stock or Cash Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Stock Purchase Plan, authorized shares for issuance		500,000
Stock option plan granted on cash award | $		$ 1.0
Cerus Corporation Inducement Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of shares of common stock reserved for future issuance					1,250,000
Share based compensation modification terms		Effective June 7, 2017, the Company no longer issues shares from the Inducement Plan.
[1] Includes the number of shares issuable under the performance-based restricted stock unit awards granted during the twelve months ended December 31, 2018.

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