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Stock-Based Compensation - Additional Information (Detail) $ in Millions |  |  | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 07, 2017 shares | Jun. 10, 2015 shares | Sep. 30, 2018 USD ($) Period shares | Dec. 31, 2017 shares | Aug. 31, 2016 shares | Jun. 12, 2013 shares
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 |  |  |
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 |  |  | 23,200,000 |  |  |
Employee Stock Purchase Plan, authorized shares for issuance |  |  |  |  |  | 19,500,000
Increase in shares of common stock authorized for issuance | 6,000,000 | 5,000,000 |  |  |  |
Stock-based compensation, award term |  |  | 10 years |  |  |
Performance-based stock awards, outstanding |  |  | 25,000 |  |  |
Outstanding options and other stock based awards |  |  | 17,622,000 | 17,138,000 |  |
Number of shares available for future issuance |  |  | 3,600,000 |  |  |
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 unvested Restricted Stock Units |  |  | 1,978,000 | 1,256,000 |  |
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. |  |  |

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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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