Life and Annuity Benefits Life and Annuity Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Liability for Future Policy Benefit, before Reinsurance [Abstract]
Increase in life and annuity benefits	$ 31.3	$ 0.0	$ 93.1	$ 0.0
Cumulative adjustment to life and annuity benefits	$ 93.1		$ 93.1

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