XML 52 R42.htm IDEA: XBRL DOCUMENT
v3.22.2.2

Stockholders' equity - Narrative (Details) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2022 | Jun. 30, 2022 | Mar. 31, 2022 | Jun. 30, 2021 | Sep. 30, 2022 | Sep. 30, 2021 | Dec. 31, 2013 | Jun. 09, 2022 | Jun. 08, 2022 | Feb. 28, 2022 | Dec. 31, 2021 | May 31, 2020
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, shares authorized (in shares) | 25,000,000 |  |  |  | 25,000,000 |  |  |  |  |  | 25,000,000 |
Preferred stock, shares issued (in shares) | 0 |  |  |  | 0 |  |  |  |  |  | 0 |
Net proceeds from public offering of common stock |  |  |  |  | $ 110,954,000 | $ 33,287,000 |  |  |  |  |  |
Number of shares into which warrant issued during period may be converted (in shares) |  |  |  |  |  |  | 129,491 |  |  |  |  |
Warrant exercise price per share (in dollars per share) |  |  |  |  |  |  | $ 0.05 |  |  |  |  |
Contractual life of warrants |  |  |  |  |  |  | 10 years |  |  |  |  |
Number of shares of common stock into which warrants may be converted (in shares) | 22,590 |  |  |  | 22,590 |  |  |  |  |  |  |
Common stock, par value (in dollars per share) | $ 0.0001 |  |  |  | $ 0.0001 |  |  | $ 0.0001 |  |  | $ 0.0001 |
Common stock, shares authorized (in shares) | 350,000,000 |  |  |  | 350,000,000 |  |  | 350,000,000 | 175,000,000 |  | 175,000,000 |
Common stock voting rights |  |  |  |  | The holders of the common stock are entitled to one vote for each share held. |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock under public offering, net of issuance costs (in shares) | 1,382,631 | 9,904,964 | 13,169,903 | 2,271,074 |  |  |  |  |  |  |  |
Exercise of common stock warrant in exchange for common stock (in shares) |  | 16,654 |  |  | 16,654 |  |  |  |  |  |  |
2020 ATM |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of shares authorized to be offered and sold |  |  |  |  |  |  |  |  |  |  |  | $ 100,000,000
2020 ATM | Common Stock |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock under public offering, net of issuance costs (in shares) |  |  | 11,740,210 |  |  |  |  |  |  |  |  |
Net proceeds from public offering of common stock |  |  | $ 54,800,000 |  |  |  |  |  |  |  |  |
2022 ATM |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of shares authorized to be offered and sold |  |  |  |  |  |  |  |  |  | $ 100,000,000 |  |
2022 ATM | Common Stock |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock under public offering, net of issuance costs (in shares) |  |  |  |  | 12,717,288 |  |  |  |  |  |  |
Net proceeds from public offering of common stock |  |  |  |  | $ 56,500,000 |  |  |  |  |  |  |
Remaining unsold and available amount | $ 42,400,000 |  |  |  | $ 42,400,000 |  |  |  |  |  |  |

X
- Definition Number of securities into which the class of warrant or right issued during the period may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares. + References No definition available. + Details
Name: | mrsn_ClassOfWarrantOrRightSecuritiesCalledByWarrantsOrRightsNumberIssued
Namespace Prefix: | mrsn_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition The contractual life of warrants. + References No definition available. + Details
Name: | mrsn_ContractualLifeOfWarrants
Namespace Prefix: | mrsn_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of common stock shares authorized to be issued under the program. + References No definition available. + Details
Name: | mrsn_StockIssuanceProgramAuthorizedAmount
Namespace Prefix: | mrsn_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Value of shares that remain unsold and available under a stock issuance program. + References No definition available. + Details
Name: | mrsn_StockIssuanceProgramRemainingUnsoldAndAvailableAmount
Namespace Prefix: | mrsn_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Shares of stock issued as a result of the exercise of warrants. + References No definition available. + Details
Name: | mrsn_WarrantsExercisedInPeriod
Namespace Prefix: | mrsn_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ClassOfStockLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Exercise price per share or per unit of warrants or rights outstanding. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI https://asc.fasb.org/extlink&oid=126973232&loc=d3e21475-112644 + Details
Name: | us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI https://asc.fasb.org/extlink&oid=126973232&loc=d3e21475-112644 + Details
Name: | us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Face amount or stated value per share of common stock. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_CommonStockParOrStatedValuePerShare
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition The maximum number of common shares permitted to be issued by an entity's charter and bylaws. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_CommonStockSharesAuthorized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Description of voting rights of common stock. Includes eligibility to vote and votes per share owned. Include also, if any, unusual voting rights. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI https://asc.fasb.org/extlink&oid=126973232&loc=d3e21475-112644 + Details
Name: | us-gaap_CommonStockVotingRights
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_PreferredStockSharesAuthorized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 13 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126973232&loc=SL123496158-112644 + Details
Name: | us-gaap_PreferredStockSharesIssued
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition The cash inflow from the additional capital contribution to the entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3255-108585 + Details
Name: | us-gaap_ProceedsFromIssuanceOfCommonStock
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of new stock issued during the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI https://asc.fasb.org/extlink&oid=126973232&loc=d3e21463-112644 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI https://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_StockIssuedDuringPeriodSharesNewIssues
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_CommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsidiarySaleOfStockAxis=mrsn_AtMarketEquityOfferingProgram2020ATMMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsidiarySaleOfStockAxis=mrsn_AtMarketEquityOfferingProgram2022ATMMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |