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License, Collaboration, and Funding Agreements - Additional Information (Detail) - USD ($) |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Oct. 31, 2018 | Feb. 28, 2017 | Dec. 31, 2016 | Aug. 31, 2014 | Dec. 31, 2019 | Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Upfront payments | $ 92,000 |  |  |  |  |  |  |
Common Stock issued |  |  |  |  |  |  | 22,000 |
Research and Development Expense |  |  |  |  |  |  | $ 0 |
Research and development expense |  |  |  |  |  | $ 5,655,000 | 4,744,000 |
AUSTRIA [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Grant income |  |  |  |  |  | 14.00% |  |
Genzyme Agreement [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Upfront payments |  |  |  | $ 50,000 |  |  |  |
Cash payment |  |  |  | $ 300,000 |  |  |  |
Common Stock issued |  |  |  | 107,371 |  |  |  |
Equity reverse stock split |  |  |  | Adjusted for the 6 for 1 reverse stock split and exchange ratio |  |  |  |
Research and Development Expense |  |  |  | $ 734,000 |  |  |  |
Shares issued price per share |  |  |  | $ 0.01 |  |  |  |
Future milestone payments |  |  |  |  |  | $ 25,000,000 |  |
Consideration paid to equity shareholders |  |  |  |  |  | 5.50% |  |
Derivative Liability |  |  |  |  |  | $ 0 |  | $ 183,000
License agreement termination description |  |  |  |  |  | The agreement may be terminated by either party with at least 90 days' notice in the event of material breach by the other party that remains uncured for 90 days, by either party for insolvency or bankruptcy of the other party, immediately by Genzyme if the Company challenges the licensed patents, or immediately by the Company if a material safety issue arises. |  |
Payment on obligation under license agreement |  |  |  |  |  | $ 0 | 0 |
Milestone payment obligation |  |  |  |  |  | 0 | 0 |
Georgetown Agreement [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
License agreement termination description |  |  |  |  | The event (i) the Company fails to pay any amount and fails to cure such failure within 30 days after receipt of notice, (ii) the Company defaults in its obligation to obtain and maintain insurance and fails to remedy such breach within 45 days after receipt of notice, or (iii) the Company declares insolvency or bankruptcy. The Company may terminate the agreement at any time upon at least 60 days' written notice. |  |  |
Payment on obligation under license agreement |  |  |  |  |  | 0 | 0 |
Milestone payment obligation |  |  |  |  |  | 0 | 0 |
One time upfront payment |  |  | $ 50,000 |  |  |  |  |
Milestone payments |  |  | 800,000 |  |  |  |  |
BIDMC Agreement [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
License agreement termination description |  |  |  |  | The event (i) the Company fails to pay any amount and fails to cure such failure within 15 days after receipt of notice, (ii) the Company is in material breach of any material provision of the agreement and fails to remedy such breach within 60 days after receipt of notice, or (iii) the Company declares insolvency or bankruptcy. We may terminate the agreement at any time upon at least 90 days' written notice. |  |  |
Payment on obligation under license agreement |  |  |  |  |  | 0 | $ 0 |
One time upfront payment |  |  | $ 20,000 |  |  |  |  |
Adimab Option Agreement [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Future milestone payments |  | $ 25,000,000 |  |  |  |  |  |
Option fee |  | 250,000 |  |  |  |  |  |
Adimab Option Agreement [Member] | Adimab, LLC [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Future milestone payments |  |  |  |  |  | 24,400,000 |  |
Option fee |  |  |  |  |  | 300,000 |  |
Research and development expense |  |  |  |  |  | 0 |  |
Adimab Collaboration Agreement | Adimab, LLC [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Research and development expense |  |  |  |  |  | 0 |  |
Research and Development Incentive [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Grant receivable |  |  |  |  |  | 1,600,000 |  |
Grant Agreement [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Qualifying expenses related to letter agreement |  |  |  |  |  | $ 0 |  |
February 2017 Grant Agreement | Gates Foundation [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Grant receivable |  | $ 9,300,000 |  |  |  |  |  |
Janssen License and Option Agreement [Member] | Janssen Pharmaceuticals Incorporation [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Payables from related parties |  |  |  |  |  |  |  | 3,500,000
Payment received from related parties |  |  |  |  |  |  |  | $ 3,500,000
License and options agreement description |  |  |  |  |  |  |  | Pursuant to the Janssen License and Option Agreement, Arsanis granted to Janssen (i) a non-exclusive license to specified patents in Arsanis’s portfolio related to the ASN200 E. coli program, and (ii) an option for Janssen to acquire these patents in the future if specified conditions are met. Janssen agreed to pay Arsanis $3.5 million within 15 business days after the December 12, 2018 effective date of the Janssen License and Option Agreement, in addition to a future $0.5 million payment in the event Janssen exercises its option to acquire the relevant patents. Arsanis received the $3.5 million payment from Janssen in December 2018.
Janssen License and Option Agreement [Member] | Janssen Pharmaceuticals Incorporation [Member] | Patents [Member] |  |  |  |  |  |  |  |
Collaboration License And Funding Arrangements [Line Items] |  |  |  |  |  |  |  |
Option to acquire the relevant patents |  |  |  |  |  |  |  | $ 500,000

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