Life And Annuity Benefits (Narrative) (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Liability for Future Policy Benefits [Abstract]
Average reserve valuation rate	2.30%
Life and annuity benefits reserve single contract concentration	33.10%
Annuities included in life and annuity benefits subject to discretionary withdrawal	$ 0

Average interest rate for traditional life contracts used to calculate future benefits to be paid to or on behalf of policyholders and related expenses less the present value of future net premiums using assumptions such as estimates of expected investment yields, mortality, morbidity, terminations and expenses, applicable at the time the insurance contracts are made and costs of settling insured claims and costs incurred in the claims settlement process of traditional life.

No definition available.

Name:	mkl_TraditionalLifeInterestRateAverage
Namespace Prefix:	mkl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant