Unpaid Losses And Loss Adjustment Expenses (Reconciliation Of Consolidated Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross reserves for losses and loss adjustment expenses, beginning of year	$ 30,857,453	$ 26,633,094
Reinsurance recoverable on unpaid losses, beginning of year	14,150,484	11,120,367
Net reserves for losses and loss adjustment expenses, beginning of year	16,706,969	15,512,727
Effect of foreign currency rate changes on beginning of year balance	(40,017)	160,363
Adjusted net reserves for losses and loss adjustment expenses, beginning of year	16,666,952	15,673,090
Incurred losses and loss adjustment expenses, current accident year	4,015,514	4,156,998
Incurred losses and loss adjustment expenses, prior accident years	(385,941)	(360,473)
Total incurred losses and loss adjustment expenses	3,629,573	3,796,525
Payments, current accident year	465,274	444,603
Payments, prior accident years	3,006,278	2,640,264
Total payments	3,471,552	3,084,867
Effect of foreign currency rate changes on current year activity	(3,206)	900
Retroactive reinsurance transaction with Hagerty Re, net losses and loss adjustment expenses	(62,202)	0
Net reserves for losses and loss adjustment expenses of Markel CATCo Re	0	(2,005)
Commutation of ceded reinsurance contracts	0	312,244
Net reserves for losses and loss adjustment expenses, end of period	16,964,911	16,695,887
Reinsurance Recoverables, Incurred but Not Reported Claims	16,895,016	12,442,082
Gross reserves for losses and loss adjustment expenses, end of period	33,859,927	29,137,969
Program Services Fronting
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for expected credit losses within fronting operations (see note 7)	$ 205,346	$ 0

The liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims, adjusted for foreign currency movements, commutations and other events. The liability includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.

No definition available.