Markel Corporation (Parent Company Only) Financial Information (Schedule Of Condensed Balance Sheets) (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, available-for-sale		$ 10,043,188	$ 9,940,670
Equity securities, available-for-sale		0	5,967,847
Equity securities		5,720,945	0
Short-term investments, available-for-sale (estimated fair value approximates cost)		1,077,696	2,160,974
Total Investments		16,841,829	18,069,491
Cash and cash equivalents		2,014,168	2,198,459
Restricted cash and cash equivalents		382,264	302,387
Income taxes receivable		49,300	64,300
Other assets		1,383,823	1,223,365
Total Assets		33,306,263	32,805,016	$ 25,875,299
Net deferred tax liability		446,045	505,579
Other liabilities		1,796,036	1,748,460
Total Liabilities		24,031,899	23,137,166
Total Shareholders' Equity		9,080,653	9,504,148
Total liabilities and equity		33,306,263	32,805,016
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, available-for-sale		717,681	532,438
Equity securities, available-for-sale		0	646,060
Equity securities	[1]	1,175,120	0
Short-term investments, available-for-sale (estimated fair value approximates cost)		451,499	1,159,323
Total Investments		2,344,300	2,337,821
Cash and cash equivalents		263,043	349,347
Restricted cash and cash equivalents		3,177	1,419
Receivables		19,295	18,684
Investments in consolidated subsidiaries		10,697,605	9,510,215
Notes receivable from subsidiaries		160,111	140,110
Income taxes receivable		21,174	5,704
Other assets		135,722	121,233
Total Assets		13,644,427	12,484,533
Senior long-term debt		2,539,389	2,537,331
Notes payable to subsidiaries	[2]	1,895,000	285,000
Net deferred tax liability		64,564	84,507
Other liabilities		64,820	73,547
Total Liabilities		4,563,773	2,980,385
Total Shareholders' Equity		9,080,654	9,504,148
Total liabilities and equity		$ 13,644,427	$ 12,484,533

[1] Effective January 1, 2018, the Company adopted ASU No. 2016-01. As a result, equity securities are no longer classified as available-for-sale with unrealized gains and losses recognized in other comprehensive income; rather, all changes in the fair value of equity securities are now recognized in net income. Prior periods have not been restated to conform to the current presentation. See note 1.

[2] 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.