Employee Benefit Plan, Description of Plan (Details) - EBP 001 [Member] - USD ($)	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
EBP, Description of Plan [Line Items]
EBP, Employer Contribution	6.00%
EBP, Employer Contribution, Participant Compensation Matched, Percentage, First 2%	100.00%
EBP, Employer Contribution, Matching Percentage, 100%	2.00%
EBP, Employer Contribution, Participant Compensation Matched, Percentage, Next 2%	50.00%
EBP, Employer Contribution, Matching Percentage, 50%	2.00%
EBP, Employer Contribution, Matching Percentage	3.00%
EBP, Participant Contribution, Pretax, Maximum Annual Compensation to Total Compensation, Percentage	50.00%
EBP, Participant Contribution, Pretax, Maximum Annual Compensation, Amount	$ 23,500	$ 23,000
EBP, Participant Catch-Up Contribution, Maximum Amount	7,500	7,500
EBP, Participant Super Catch-Up Contribution, Maximum Amount	$ 11,250
EBP, Participant Contribution, Automatic, Deferral Rate	4.00%
EBP, Participant Contribution, Automatic Increase	1.00%
EBP, Participant Contribution, Maximum Contribution Rate	10.00%
EBP, Participant Contribution, 0% Election Deferral Rate	0.00%
EBP, Employer Contribution, Vesting Percentage, Less than two years of service	0.00%
EBP, Employer Contribution, Vesting Percentage, Two years of service	20.00%
EBP, Employer Contribution, Vesting Percentage, Three years of service	50.00%
EBP, Employer Contribution, Vesting Percentage, Four or more years of service	100.00%
EBP, Forfeited Nonvested Account	$ 1,403,066	1,134,688
EBP, Forfeited Nonvested Account, Decrease from Employer Contribution	1,329,071	1,324,841
EBP, Forfeited Nonvested Account, Decrease from Administrative Expense	97,439	$ 86,905
EBP, Forfeited Nonvested Account, Unused at year end	$ 71,878
EBP, Employer Contribution, Vesting Percentage, Plan termination	100.00%
EBP, Note Receivable from Participant, Account, Minimum Borrowing, Amount	$ 1,000
EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount	$ 50,000
EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage	50.00%

EBP, Employer Contribution

No definition available.

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Balance Type:	na
Period Type:	duration

- Definition

EBP, Employer Contribution, Matching Percentage, 100%

+ References

No definition available.

+ Details

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Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Employer Contribution, Matching Percentage, 50%

+ References

No definition available.

+ Details

Name:	mkl_EBPEmployerContributionMatchingPercentage50
Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Employer Contribution, Participant Compensation Matched, Percentage, First 2%

+ References

No definition available.

+ Details

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Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Employer Contribution, Participant Compensation Matched, Percentage, Next 2%

+ References

No definition available.

+ Details

Name:	mkl_EBPEmployerContributionParticipantCompensationMatchedPercentageNext2
Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Employer Contribution, Vesting Percentage, Four or more years of service

+ References

No definition available.

+ Details

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Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Employer Contribution, Vesting Percentage, Less than two years of service

+ References

No definition available.

+ Details

Name:	mkl_EBPEmployerContributionVestingPercentageLessThanTwoYearsOfService
Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Employer Contribution, Vesting Percentage, Plan termination

+ References

No definition available.

+ Details

Name:	mkl_EBPEmployerContributionVestingPercentagePlanTermination
Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Employer Contribution, Vesting Percentage, Three years of service

+ References

No definition available.

+ Details

Name:	mkl_EBPEmployerContributionVestingPercentageThreeYearsOfService
Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Employer Contribution, Vesting Percentage, Two years of service

+ References

No definition available.

+ Details

Name:	mkl_EBPEmployerContributionVestingPercentageTwoYearsOfService
Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Forfeited Nonvested Account, Unused at year end

+ References

No definition available.

+ Details

Name:	mkl_EBPForfeitedNonvestedAccountUnusedAtYearEnd
Namespace Prefix:	mkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

EBP, Participant Catch-Up Contribution, Maximum Amount

+ References

No definition available.

+ Details

Name:	mkl_EBPParticipantCatchUpContributionMaximumAmount
Namespace Prefix:	mkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

EBP, Participant Contribution, 0% Election Deferral Rate

+ References

No definition available.

+ Details

Name:	mkl_EBPParticipantContribution0ElectionDeferralRate
Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Participant Contribution, Automatic Increase

+ References

No definition available.

+ Details

Name:	mkl_EBPParticipantContributionAutomaticIncrease
Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Participant Contribution, Maximum Contribution Rate

+ References

No definition available.

+ Details

Name:	mkl_EBPParticipantContributionMaximumContributionRate
Namespace Prefix:	mkl_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Participant Super Catch-Up Contribution, Maximum Amount

+ References

No definition available.

+ Details

Name:	mkl_EBPParticipantSuperCatchUpContributionMaximumAmount
Namespace Prefix:	mkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of employer matching contribution on eligible participant contribution to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanEmployerContributionMatchingPercentage
Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of plan asset, in forfeited nonvested account, no longer allocated to participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanForfeitedNonvestedAccount
Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of decrease in forfeited nonvested account of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) from administrative expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-04(5)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of decrease in forfeited nonvested account of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) from employer contribution.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 962
-SubTopic 325
-Name Accounting Standards Codification
-Section 55
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478873/962-325-55-17

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-04(5)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of maximum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow.

+ References

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMaximumBorrowingAmount
Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of maximum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow to total participant account balance.

+ References

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMaximumBorrowingToTotalAccountPercentage
Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of minimum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow.

+ References

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMinimumBorrowingAmount
Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of eligible compensation contributed for participant automatically enrolled in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanParticipantContributionAutomaticDeferralRate
Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of maximum pretax annual compensation that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), is permitted to contribute.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage of maximum pretax annual compensation that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), is permitted to contribute to total pretax annual compensation.

+ References

+ Details

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- Details

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