Unpaid Losses And Loss Adjustment Expenses (Reconciliation Of Consolidated Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Gross reserves for losses and loss adjustment expenses, beginning of year			$ 23,483,321	$ 20,947,898
Reinsurance recoverable on unpaid losses, beginning of year			8,820,567	7,994,884
Net reserves for losses and loss adjustment expenses, beginning of year			14,662,754	12,953,014
Effect of foreign currency rate changes on beginning of year balance	$ 45,503	$ 16,246	45,503	16,246
Adjusted net reserves for losses and loss adjustment expenses, beginning of year			14,708,257	12,969,260
Incurred losses and loss adjustment expenses, current accident year	1,413,526	1,434,871	4,155,146	3,934,969
Incurred losses and loss adjustment expenses, prior accident years			(344,212)	(169,754)
Total incurred losses and loss adjustment expenses			3,810,934	3,765,215
Payments, current accident year			444,221	419,998
Payments, prior accident years			2,415,104	2,018,199
Total payments			2,859,325	2,438,197
Effect of foreign currency rate changes on current year activity			1,861	416
Net reserves for losses and loss adjustment expenses of Markel CATCo Re			(78,091)	(146,895)
Reinsurance recoverable for retroactive reinsurance transaction			0	(125,067)
Net reserves for losses and loss adjustment expenses, end of period	15,583,636	14,024,732	15,583,636	14,024,732
Reinsurance recoverable on unpaid losses	10,293,842	8,119,634	10,293,842	8,119,634
Gross reserves for losses and loss adjustment expenses, end of period	$ 25,877,478	$ 22,144,366	$ 25,877,478	$ 22,144,366