Retirement Benefits (Reconciliation of Prior Service Credit) (Details) - Non-U.S. - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
Pension Benefits
Reconciliation of prior service (cost) credit recognized in accumulated other comprehensive (loss) income:
Beginning balance	$ (14)	$ (17)
Recognized as component of net benefit (credit) cost:
Total recognized as component of net benefit cost (credit)	1	1
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Plan amendments	0	2
Exchange rate adjustments	(1)	0
Ending balance	(14)	(14)
Post-retirement Benefits
Reconciliation of prior service (cost) credit recognized in accumulated other comprehensive (loss) income:
Beginning balance	1	3
Recognized as component of net benefit (credit) cost:
Total recognized as component of net benefit cost (credit)	(1)	(2)
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Plan amendments	0	0
Exchange rate adjustments	0	0
Ending balance	$ 0	$ 1

Accumulated Other Comprehensive Income (Loss) [Roll Forward]

No definition available.

Name:	mrsh_AccumulatedOtherComprehensiveIncomeLossRollForward
Namespace Prefix:	mrsh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Other Comprehensive Income (Loss), Defined Benefit Plan, Prior Service Cost (Credit), Foreign Currency Exchange Adjustments Arising During Period, Before Tax

+ References

No definition available.

+ Details

Name:	mrsh_OtherComprehensiveIncomeLossDefinedBenefitPlanPriorServiceCostCreditForeignCurrencyExchangeAdjustmentsArisingDuringPeriodBeforeTax
Namespace Prefix:	mrsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Other Comprehensive Income (Loss), Defined Benefit Plans Adjustment, Plan Amendment Before Tax

+ References

No definition available.

+ Details

Name:	mrsh_OtherComprehensiveIncomeLossDefinedBenefitPlansAdjustmentPlanAmendmentBeforeTax
Namespace Prefix:	mrsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-14A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-5

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

Name:	us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before tax, of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of defined benefit plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 15
-SubTopic 10
-Topic 220
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-15

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-11

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-5

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 17A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-17A

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossBeforeTaxPeriodChangeAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=us-gaap_ForeignPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: