XML 75 R62.htm IDEA: XBRL DOCUMENT v3.20.2

Recent Developments Related to COVID-19 Assets (Narrative) (Details) $ in Billions	Jun. 30, 2020 USD ($)
Assets [Abstract]
Goodwill and intangible assets	$ 4.5

X

- Definition

Total of goodwill and intangible assets

+ References

No definition available.

+ Details

Name:	mkl_GoodwillandIntangibleAssets
Namespace Prefix:	mkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- References

No definition available.

+ Details

Name:	us-gaap_AssetsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration