Label	Element	Value
Noncontrolling Interest [Member]
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	$ 0
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	(2,567,000)
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	0
AOCI Attributable to Parent [Member]
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	(2,595,484,000)
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	152,940,000
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	402,853,000
Common Stock [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	$ 3,381,834,000
Shares, Issued	us-gaap_SharesIssued	13,904,000
Parent [Member]
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	$ 0
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	325,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	9,504,473,000
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	0
Retained Earnings [Member]
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	2,595,484,000
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	325,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	5,969,699,000
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	(402,853,000)
Redeemable Noncontrolling Interests, Temporary Equity [Member]
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	0
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	166,269,000
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	0
Total Equity [Member]
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	0
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	325,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	9,501,906,000
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	$ 0