Recent Developments Related to COVID-19 Income Statement (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Net investment gains (losses)	$ (1,681,441)	$ 612,191

The net gains or losses on investments during the period, also including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

No definition available.

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