Markel Corporation (Parent Company Only) Financial Information (Schedule Of Condensed Balance Sheets) (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, available-for-sale		$ 9,970,909	$ 10,043,188
Equity securities		7,590,755	5,720,945
Short-term investments, available-for-sale (estimated fair value approximates cost)		1,196,248	1,077,696
Total Investments		18,757,912	16,841,829
Cash and cash equivalents		3,072,807	2,014,168
Restricted cash and cash equivalents		427,546	382,264
Income taxes receivable		6,300	49,300
Other assets		1,906,115	1,383,823
Total Assets		37,473,815	33,306,263	$ 32,805,016
Net deferred tax liability		860,720	446,045
Other liabilities		2,504,802	1,796,036
Total Liabilities		26,217,837	24,031,899
Total Shareholders' Equity		11,070,867	9,080,653
Total liabilities and equity		37,473,815	33,306,263
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, available-for-sale		676,307	717,681
Equity securities		1,831,333	1,175,120
Short-term investments, available-for-sale (estimated fair value approximates cost)		724,558	451,499
Total Investments		3,232,198	2,344,300
Cash and cash equivalents		737,072	263,043
Restricted cash and cash equivalents		1,077	3,177
Receivables		19,846	19,295
Investments in consolidated subsidiaries		12,239,086	10,697,605
Notes receivable from subsidiaries		60,111	160,111
Income taxes receivable		2,170	21,174
Other assets		347,023	135,722
Total Assets		16,638,583	13,644,427
Senior long-term debt		3,438,910	2,539,389
Notes payable to subsidiaries	[1]	1,835,000	1,895,000
Taxes Payable		18,270	0
Net deferred tax liability		129,835	64,564
Other liabilities		145,701	64,820
Total Liabilities		5,567,716	4,563,773
Total Shareholders' Equity		11,070,867	9,080,654
Total liabilities and equity		$ 16,638,583	13,644,427
Markel Global Reinsurance Company
Condensed Financial Statements, Captions [Line Items]
Notes payable to subsidiaries	[1]		$ 1,400,000

[1] In December 2018, Markel Corporation purchased Markel Global Reinsurance Company, an indirectly owned subsidiary of Markel Corporation, from Alterra USA Holdings Limited, another indirectly owned subsidiary of Markel Corporation, by issuing a $1.4 billion note payable to subsidiary.