Life and Annuity Benefits Life and Annuity Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Liability for Future Policy Benefit, before Reinsurance [Abstract]
Increase in life and annuity benefits	$ 36.0	$ 0.0	$ 61.8	$ 0.0
Cumulative adjustment to life and annuity benefits	$ 61.8		$ 61.8

The adjustment to life and annuity benefits during the period attributable to the change in unrealized gains on the underlying investment portfolio during the period.

No definition available.

Name:	mkl_AdjustmenttoLifeandAnnuityBenefits
Namespace Prefix:	mkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration