Debt - Additional Information (Detail)			1 Months Ended	9 Months Ended
	Apr. 27, 2017	Dec. 30, 2016 USD ($)	Jul. 31, 2017 USD ($) Tranche	Sep. 30, 2018	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)
Cerus Term Loans
Debt Instrument [Line Items]
Term loan, face amount		$ 20,000,000
Term of agreement		5 years
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
Terms of required periodic payments of interest and principal				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 each of these amendments to the Term Loan Agreement resulted in a debt modification. As a result, the accounting treatment for the Term Loan continued 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.
Final payment term percent	7.00%
Cerus Amended Credit Agreement | 2017 Term Loans
Debt Instrument [Line Items]
Term loan, face amount			$ 40,000,000
Interest rate, description				The 2017 Term Loan A bears 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 interest rate on the 2017 Term Loan A at September 30, 2018, was approximately 9.12%. The Company will also be required to make a final payment fee of 8.00% of the principal amount of the 2017 Term Loan A. The Amended Credit Agreement contains certain nonfinancial covenants, with which the Company was in compliance at September 30, 2018.
Percentage of investments made in subsidiary			35.00%
Number of loan tranches | Tranche			2
Repay in full all of outstanding term loans			$ 1,400,000
Terms of required periodic payments of interest and principal				The Amended Credit Agreement provided 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 were 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 and the final payment of the Term Loan Agreement of $1.4 million. The availability of the second tranche of $10.0 million (“2017 Term Loan B”) expired on May 14, 2018, and the Company did not elect to draw the 2017 Term Loan B. The 2017 Term Loan A requires interest-only payments through February 1, 2019, followed by 42 monthly payments of equal principal plus declining interest payments.
Cerus Amended Credit Agreement | 2017 Term Loans | Three-month U.S. LIBOR
Debt Instrument [Line Items]
Debt, applicable margin			6.72%
First Tranche (Term Loan A) | Cerus Term Loans
Debt Instrument [Line Items]
Term loan, face amount						$ 10,000,000
Interest rate						6.95%
Maturity period				Jun. 01, 2019
First Tranche (Term Loan A) | Cerus Amended Credit Agreement | 2017 Term Loans
Debt Instrument [Line Items]
Term loan, face amount			$ 30,000,000
Interest rate				9.12%
Final payment term percent			8.00%
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
First Tranche (Term Loan A) | Cerus Amended Credit Agreement | 2017 Term Loans | Minimum
Debt Instrument [Line Items]
Debt instrument floating interest rate percentage			8.01%
Second Tranche (Term Loan B) | Cerus Term Loans
Debt Instrument [Line Items]
Term loan, face amount					$ 10,000,000
Borrowing conditions				The second tranche of $10.0 million (“Term Loan B”) was drawn in June 2015.
Interest rate					7.01%
Maturity period				Jun. 01, 2019
Second Tranche (Term Loan B) | Cerus Amended Credit Agreement | 2017 Term Loans
Debt Instrument [Line Items]
Loan and security agreement available upon revenue achievement			$ 10,000,000
Expiration date to draw second tranche			May 14, 2018