XML 54 R44.htm IDEA: XBRL DOCUMENT
v3.22.2.2

Stock-based compensation - Plans Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 01, 2022 | Jun. 30, 2017 | Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2022 | Sep. 30, 2021 | Feb. 28, 2022 | Dec. 31, 2021
Stock options |  |  |  |  |  |  |  |
Number of shares outstanding (in shares) |  |  | 10,524,780 |  | 10,524,780 |  |  | 8,342,429
Weighted-average grant date fair value of options granted (in dollars per share) |  |  |  |  | $ 3.96 | $ 12.37 |  |
Share-Based compensation arrangement by share-based payment award, options, exercises in period, intrinsic value |  |  |  |  | $ 100 | $ 3,900 |  |
Cash received from the exercise of stock options |  |  |  |  | $ 206 | $ 1,545 |  |
Expected dividend yield |  |  | 0.00% | 0.00% | 0.00% | 0.00% |  |
Stock options |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Unrecognized stock compensation cost |  |  | $ 34,500 |  | $ 34,500 |  |  |
Weighted-average amortization period of unrecognized stock compensation cost |  |  |  |  | 2 years 2 months 12 days |  |  |
Restricted stock units |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Unrecognized stock compensation cost |  |  | $ 12,600 |  | $ 12,600 |  |  |
Weighted-average amortization period of unrecognized stock compensation cost |  |  |  |  | 2 years 9 months 18 days |  |  |
Restricted stock units | Maximum |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Vesting period |  |  |  |  | 4 years |  |  |
Restricted stock units | Minimum |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Vesting period |  |  |  |  | 1 year |  |  |
2007 Plan |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Number of shares outstanding (in shares) |  | 3,141,625 |  |  |  |  |  |
2017 Plan |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Number of shares authorized (in shares) |  | 2,255,000 |  |  |  |  |  |
Cumulative annual increase in number of shares issuable |  | 4.00% |  |  |  |  |  |
Additional shares authorized (in shares) | 2,948,362 |  |  |  |  |  |  |
Number of shares available for future issuance (in shares) |  |  | 1,372,006 |  | 1,372,006 |  |  |
Number of options granted in period (in shares) |  |  |  |  | 3,532,433 |  |  |
2017 Plan | Maximum |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Expiration period |  |  |  |  | 10 years |  |  |
2017 Plan | Stock options |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Vesting period |  |  |  |  | 4 years |  |  |
Inducement award program |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Number of shares authorized (in shares) |  |  |  |  |  |  | 2,000,000 |
Number of shares available for future issuance (in shares) |  |  | 1,403,975 |  | 1,403,975 |  |  |
Number of options granted in period (in shares) |  |  |  |  | 612,450 |  |  |
Inducement award prior to inducement award program |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Number of shares outstanding (in shares) |  |  | 757,500 |  | 757,500 |  |  |
2017 ESPP |  |  |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |
Number of shares available for future issuance (in shares) |  |  | 412,330 |  | 412,330 |  |  |
Purchase of common stock under ESPP (in shares) |  |  |  |  | 154,235 | 36,198 |  |

X
- Definition Maximum number of shares that may be cumulatively increased annually in accordance with the plan, as a percentage of outstanding capital stock. + References No definition available. + Details
Name: | mrsn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximumCumulativeAnnualIncrease
Namespace Prefix: | mrsn_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of grants made during the period for share options (or share units) and for other than stock (or unit) options, including but not limited to, restricted stock options (or units). This amount does not include options granted as part of an inducement award. + References No definition available. + Details
Name: | mrsn_ShareBasedCompensationArrangementsByShareBasedPaymentAwardGrantsInPeriodGross
Namespace Prefix: | mrsn_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
Name: | us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cost to be recognized for option under share-based payment arrangement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
Name: | us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of cash inflow from exercise of option under share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2A -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=128089324&loc=SL79508275-113901 Reference 2: 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_ProceedsFromStockOptionsExercised
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of additional shares authorized for issuance under share-based payment arrangement. + References No definition available. + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of shares authorized for issuance under share-based payment arrangement. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- 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 https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of options outstanding, including both vested and non-vested options. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(ii) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
Name: | us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of shares issued during the period as a result of an employee stock purchase plan. + References Reference 1: 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 2: 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 3: 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 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(29)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PlanNameAxis=mrsn_StockIncentivePlan2007Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PlanNameAxis=mrsn_StockIncentivePlan2017Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PlanNameAxis=mrsn_InducementAwardProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PlanNameAxis=mrsn_InducementAwardPriorToInducementAwardProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PlanNameAxis=mrsn_A2017ESPPMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |