Life And Annuity Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Liability for Future Policy Benefits [Abstract]
Adjustment to life and annuity benefits	$ 0.0	$ 47.9	$ 0.0	$ 47.9

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