Debt - Additional Information (Detail)				1 Months Ended	9 Months Ended
	Apr. 27, 2017	Jul. 28, 2016	Jun. 30, 2014 USD ($) Tranche	Jul. 31, 2017 USD ($) Tranche	Sep. 30, 2017
Cerus Term Loans
Line of Credit Facility [Line Items]
Maximum borrowing limit			$ 30,000,000
Term of agreement			5 years
Number of loan tranches | Tranche			3
Interest rate, description					Term Loan A bore an interest rate of 6.95%. Term Loan B bore an interest rate of 7.01%.
Principal and interest payments	18 months	29 months	36 months
Final payment term percent			7.00%
Terms of required periodic payments of interest and principal					On September 29, 2015, the Term Loan Agreement was amended to extend (i) the period in which the third tranche could have been drawn and (ii) the interest-only period for all advances under the Term Loan Agreement. The Company was required to make interest only payments through June 2016, followed by thirty-six months of equal principal and interest payments thereafter. On July 28, 2016, the Term Loan Agreement was amended to include an additional interest-only period for all advances under the Term Loan Agreement. As amended, the Company was required to make interest only payments from August 2016 through January 2017, followed by twenty-nine months of equal principal and interest payments thereafter. On April 27, 2017, the Term Loan Agreement was amended to include an additional interest-only period for all advances under the Term Loan Agreement. As amended, the Company was required to make interest only payments from May 2017 through December 2017, followed by eighteen months of equal principal and interest payments thereafter. The Company determined that these amendments to the Term Loan Agreement resulted in debt modifications. As a result, the accounting treatment for the Term Loan continues under the interest method, with a new effective interest rate based on revised cash flows calculated on a prospective basis upon the execution of each of these amendments to the Term Loan Agreement. The Company was also required to make a final payment equal to 7% of the principal amounts of the Term Loans drawn payable on the earlier to occur of maturity or prepayment.
Cerus Amended Credit Agreement | 2017 Term Loans
Line of Credit Facility [Line Items]
Maximum borrowing limit				$ 40,000,000
Number of loan tranches | Tranche				2
Borrowing conditions					However, if the Company draws the 2017 Term Loan B, then the interest-only period will be extended through August 1, 2019, and the amortization period will be reduced to 36 months.
Interest rate, description					Interest on 2017 Term Loan A and 2017 Term Loan B will bear interest at a rate equal to the greater of (i) 8.01% and (ii) the three-month U.S. LIBOR rate plus 6.72%. The Company will also be required to make a final payment fee of 8.00% of the principal amounts of the 2017 Term Loans. The Amended Credit Agreement contains certain nonfinancial covenants, with which the Company was in compliance at September 30, 2017.
Final payment term percent				8.00%
Terms of required periodic payments of interest and principal					The Amended Credit Agreement provides for secured growth capital term loans of up to $40.0 million (the “2017 Term Loans”). All of the Company’s current and future assets, excluding its intellectual property and 35% of the Company’s investment in Cerus Europe B.V., are secured for its borrowings under the Amended Credit Agreement. The 2017 Term Loans are available in two tranches. The first tranche of $30.0 million (“2017 Term Loan A”) was drawn by the Company on July 31, 2017, with the proceeds used in part to repay in full all of the outstanding term loans under the Term Loan Agreement of $17.6 million. The second tranche of $10.0 million (“2017 Term Loan B”) will be made available to the Company upon the Company’s achieving consolidated trailing six-month revenues as defined in the agreement (the “Revenue Milestone”). If the Revenue Milestone is achieved, the Company may draw the 2017 Term Loan B through the earlier of (i) January 31, 2019, and (ii) the date which is 60 days after the achievement of the Revenue Milestone. The 2017 Term Loans require interest-only payments through February 1, 2019, followed by 42 months payments of equal principal plus declining interest payments.
Cerus Amended Credit Agreement | 2017 Term Loans | Three-month U.S. LIBOR
Line of Credit Facility [Line Items]
Debt, applicable margin				6.72%
Cerus Amended Credit Agreement | 2017 Term Loans | Minimum
Line of Credit Facility [Line Items]
Debt instrument floating interest rate percentage				8.01%
First Tranche (Term Loan A) | Cerus Term Loans
Line of Credit Facility [Line Items]
Loan and security agreement			$ 10,000,000
Interest rate			6.95%
Maturity period					Jun. 01, 2019
First Tranche (Term Loan A) | Cerus Amended Credit Agreement | 2017 Term Loans
Line of Credit Facility [Line Items]
Loan and security agreement				$ 30,000,000
Repay in full all of outstanding term loans				$ 17,600,000
Interest-only payments date				Feb. 01, 2019
Principal plus declining interest payments				42 months
Second Tranche (Term Loan B) | Cerus Term Loans
Line of Credit Facility [Line Items]
Loan and security agreement			$ 10,000,000
Borrowing conditions					The second tranche of $10.0 million (“Term Loan B”) was drawn on June 15, 2015.
Interest rate					7.01%
Maturity period					Jun. 01, 2019
Second Tranche (Term Loan B) | Cerus Amended Credit Agreement | 2017 Term Loans
Line of Credit Facility [Line Items]
Loan and security agreement available upon revenue achievement				$ 10,000,000
Ending date to avail loan upon achievement of milestone.				Jan. 31, 2019
Number of days to avail loan upon achievement of milestone				60 days
Interest-only period extension date				Aug. 01, 2019
Amortization Period				36 months
Securities Pledged as Collateral | Cerus Term Loans
Line of Credit Facility [Line Items]
Percentage of investments made in subsidiary			35.00%
Securities Pledged as Collateral | Cerus Amended Credit Agreement | 2017 Term Loans
Line of Credit Facility [Line Items]
Percentage of investments made in subsidiary				35.00%