Variable Interest Entities (Markel CATCo Re Balance Sheet) (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Variable Interest Entity [Line Items]
Cash and cash equivalents	$ 3,873,041	$ 3,747,060
Restricted cash and cash equivalents	753,476	584,974
Total Assets	62,283,238	55,045,710
Unpaid losses and loss adjustment expenses	25,877,478	23,483,321	$ 22,144,366	$ 20,947,898
Other liabilities	4,427,425	3,927,498
Total Liabilities	44,620,410	39,519,817
Shareholders' equity	17,014,239	14,983,928
Noncontrolling interests	133,096	72,280
Total Equity	17,147,335	15,056,208
Total Liabilities and Equity	62,283,238	55,045,710
Markel CATCo Re
Variable Interest Entity [Line Items]
Cash and cash equivalents	125,700	91,301
Restricted cash and cash equivalents	123,431	173,800
Other assets and receivables due from cedents	16,327	19,292
Total Assets	265,458	284,393
Unpaid losses and loss adjustment expenses	106,876	184,967
Other liabilities	1,076	1,842
Total Liabilities	107,952	186,809
Shareholders' equity	21,139	21,139
Noncontrolling interests	136,367	76,445
Total Equity	157,506	97,584
Total Liabilities and Equity	$ 265,458	$ 284,393