Markel Corporation (Parent Company Only) Financial Information (Schedule Of Condensed Statements Of Cash Flows) (Details) - USD ($) $ in Thousands	3 Months Ended																								12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016	[1]	Jun. 30, 2016	[1]	Mar. 31, 2016		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) to Shareholders	$ (751,543)	[1]	$ 409,438		$ 278,231		$ (64,306)	[1]	$ 434,881	[1]	$ (259,141)		$ 149,660		$ 69,869	[1]	$ 132,726	[1]	$ 83,796		$ 78,797		$ 160,370	[1]	$ (128,180)	[2]	$ 395,269	[2]	$ 455,689	[2]
Net Cash Provided By Operating Activities																									892,857		858,529		534,623
Proceeds from sales of fixed maturities and equity securities																									419,199		577,650		365,822
Proceeds from maturities, calls and prepayments of fixed maturities																									551,808		1,129,895		963,165
Cost of fixed maturities and equity securities purchased																									(1,545,913)		(1,176,281)		(2,205,939)
Net change in short-term investments																									1,101,636		234,743		(689,194)
Acquisitions, net of cash acquired																									(1,175,211)		(1,431,712)		(7,527)
Additions to property and equipment																									(106,593)		(74,652)		(63,674)
Other																									(42,165)		(4,100)		(1,134)
Net Cash Provided (Used) By Investing Activities																									(797,239)		(744,457)		(1,638,481)
Additions to senior long-term debt																									206,949		664,657		559,300
Premiums and fees related to early extinguishment of debt																									0		0		(43,691)
Repurchases of common stock																									(54,007)		(110,838)		(51,142)
Other																									(4,127)		(6,281)		(10,750)
Net Cash Provided (Used) By Financing Activities																									(178,985)		256,315		152,019
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents																									(104,414)		415,682		(984,977)
Cash, cash equivalents, restricted cash and restricted cash equivalents at beginning of year							2,500,846								2,085,164								3,070,141		2,500,846		2,085,164		3,070,141
CASH, CASH EQUIVALENTS, RESTRICTED CASH AND RESTRICTED CASH EQUIVALENTS AT END OF YEAR	2,396,432								2,500,846								2,085,164								2,396,432		2,500,846		2,085,164
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) to Shareholders																									(128,180)		395,269		455,689
Adjustments to reconcile net income (loss) to shareholders to net cash provided (used) by operating activities																									3,637		(166,132)		(120,564)
Net Cash Provided By Operating Activities																									(124,543)		229,137		335,125
Proceeds from sales of fixed maturities and equity securities																									204,478		20,562		1,831
Proceeds from maturities, calls and prepayments of fixed maturities																									34,560		64,705		11,960
Cost of fixed maturities and equity securities purchased																									(26,336)		(72,910)		(29,110)
Net change in short-term investments																									930,876		649,181		(731,389)
Return of capital from subsidiaries																									12,712		45,225		21,021
Decrease (increase) in notes receivable due from subsidiaries																									(20,000)		(58)		92,530
Capital contributions to subsidiaries																									(103,133)		(270,623)		0
Acquisitions, net of cash acquired																									(972,619)		(1,153,683)		0
Cost of equity method investments																									(5,117)		(10,633)		(3,100)
Additions to property and equipment																									(3,191)		0		(584)
Other																									(5,261)		6,972		(3,207)
Net Cash Provided (Used) By Investing Activities																									46,969		(721,262)		(640,048)
Additions to senior long-term debt																									0		592,923		493,149
IncreaseDecreaseInNotesPayableFromSubsidiaries																									47,105		0		0
Repayment and retirement of senior long-term debt																									0		0		(183,343)
Premiums and fees related to early extinguishment of debt																									0		0		(43,691)
Repurchases of common stock																									(54,007)		(110,838)		(51,142)
Other																									(70)		(9,848)		(337)
Net Cash Provided (Used) By Financing Activities																									(6,972)		472,237		214,636
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents																									(84,546)		(19,888)		(90,287)
Cash, cash equivalents, restricted cash and restricted cash equivalents at beginning of year							$ 350,766								$ 370,654								$ 460,941		350,766		370,654		460,941
CASH, CASH EQUIVALENTS, RESTRICTED CASH AND RESTRICTED CASH EQUIVALENTS AT END OF YEAR	$ 266,220								$ 350,766								$ 370,654								$ 266,220		$ 350,766		$ 370,654

[1] Effective January 1, 2018, the Company adopted ASU No. 2016-01. As a result, the change in fair value of equity securities is no longer 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] Effective January 1, 2018, the Company adopted ASU No. 2016-01 and equity securities are no longer classified as available-for-sale with unrealized gains and losses recognized in other comprehensive income, rather, 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.