Variable Interest Entities (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Sep. 30, 2022
Variable Interest Entity [Line Items]
Cash funding used to purchase Markel CATCo Funds' interests in Markel CATCo Re							$ 45,100
Returned capital from Markel CATCo Re	$ 24,900
Investment in Markel CATCo Re		$ 20,100		$ 20,100		$ 20,100
Percentage of Markel CATCo Re net equity held by the Company		13.00%		13.00%		23.00%
Favorable development on prior years' loss reserves				$ 344,212	$ 169,754
Redemption of Markel CATCo Re noncontrolling interests				0	(88,997)
Tail risk cover in Markel CATCo transaction		$ 95,000		95,000
Noncontrolling Interests [Member]
Variable Interest Entity [Line Items]
Redemption of Markel CATCo Re noncontrolling interests					(62,646)
Markel CATCo Re
Variable Interest Entity [Line Items]
Favorable development on prior years' loss reserves		$ (33,400)	$ (2,200)	$ (57,500)	$ (55,700)