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Label	Element	Value
Noncontrolling Interest [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	$ 7,549,000
Cumulative Effect of Adoption of ASU No 2016-13, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201613NetofTaxes	0
Common Stock [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	$ 3,404,919,000
Shares, Issued	us-gaap_SharesIssued	13,794,000
Retained Earnings [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	$ 7,453,349,000
AOCI Attributable to Parent [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	208,772,000
Cumulative Effect of Adoption of ASU No 2016-13, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201613NetofTaxes	0
Parent [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	11,067,040,000
Cumulative Effect of Adoption of ASU No 2016-13, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201613NetofTaxes	(3,827,000)
Redeemable Noncontrolling Interests, Temporary Equity [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	177,562,000
Cumulative Effect of Adoption of ASU No 2016-13, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201613NetofTaxes	0
Total Equity [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	11,074,589,000
Cumulative Effect of Adoption of ASU No 2016-13, Net of Taxes	mkl_CumulativeEffectofAdoptionofASUNo201613NetofTaxes	(3,827,000)
Preferred Stock [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	$ 0
Shares, Issued	us-gaap_SharesIssued	0

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- Definition

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- Definition

Number of shares of stock issued as of the balance sheet date, including shares that had been issued and were previously outstanding but which are now held in the treasury.

+ 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 http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

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