Label	Element	Value
AOCI Attributable to Parent [Member]
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	$ 0
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	401,539,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	133,868,000
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	$ (2,613,242,000)
Common Stock [Member]
Shares, Issued	us-gaap_SharesIssued	13,904,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	$ 3,381,834,000
Noncontrolling Interest [Member]
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	0
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	(2,567,000)
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	0
Parent [Member]
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	325,000
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	9,504,473,000
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	0
Retained Earnings [Member]
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	325,000
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	(401,539,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	5,988,771,000
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	2,613,242,000
Redeemable Noncontrolling Interests, Temporary Equity [Member]
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	0
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	166,269,000
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	0
Total Equity [Member]
Cumulative Effect of Adoption of ASU No 2014-09, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201409NetofTaxes	325,000
Cumulative Effect of Adoption of ASU No 2018-02	mkl_CumulativeEffectofAdoptionofASUNo201802	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	9,501,906,000
Cumulative Effect of Adoption of ASU No. 2016-01, Net of Tax	mkl_CumulativeEffectofAdoptionofASUNo.201601NetofTax	$ 0