Pensions and Other Postretirement Plans (Narrative) (Details)	1 Months Ended	3 Months Ended					12 Months Ended
	Jan. 31, 2022 USD ($)	Dec. 31, 2023 USD ($) investment country	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) investment	Jun. 30, 2021 USD ($)	Dec. 31, 2023 USD ($) investment country multiemployer_plan	Dec. 31, 2022 USD ($) investment multiemployer_plan	Dec. 31, 2021 USD ($) multiemployer_plan
Retirement Benefits Disclosure [Line Items]
Service cost							$ 33,787,000	$ 32,567,000	$ 22,991,000
Number of multiemployer plans contributed to | multiemployer_plan							1	1	1
Contributions to multiemployer pension plans							$ 100,000	$ 100,000	$ 100,000
Expense associated with the retirement benefits provided under incentive savings plans							13,000,000	11,600,000	10,900,000
Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Special separation benefit expense							9,886,000	3,624,000	1,132,000
Service cost							33,787,000	32,567,000	22,991,000
Accumulated benefit obligation		$ 880,300,000			$ 843,600,000		880,300,000	843,600,000
Company contributions							0	0
Estimated employer contributions in next fiscal year		$ 0					0
Benefits paid							$ 82,455,000	$ 65,240,000
Percent of Plan Assets Managed Internally by Company							40.00%
Percent Of Plan Assets Managed By Investment Companies							60.00%
Number of investment companies actively managing plan assets | country		2					2
Percentage of total plan assets		100.00%			100.00%		100.00%	100.00%
Pension Plans [Member] | Markel Corporation [Member]
Retirement Benefits Disclosure [Line Items]
Value of investments		$ 82,400,000					$ 82,400,000
Supplemental Executive Retirement Plan (SERP) [Member]
Retirement Benefits Disclosure [Line Items]
Service cost							593,000	$ 911,000	1,022,000
Accumulated benefit obligation		$ 89,400,000			$ 88,000,000		89,400,000	88,000,000
Benefits paid							6,300,000	5,900,000
Other Postretirement Plans [Member]
Retirement Benefits Disclosure [Line Items]
Benefits paid							200,000	200,000
Settlement of obligation							1,679,000	0
Settlement gain							$ 1,087,000	$ 0	$ 120,000
Discount rate to determine benefit obligation		4.53%			4.76%		4.53%	4.76%
Discount rate to determine periodic cost							4.76%	2.23%	1.78%
Other Postretirement Plans [Member] | Pre-Age 65 [Member]
Retirement Benefits Disclosure [Line Items]
Assumed health care cost trend rate		7.50%					7.50%
Direction of change for assumed health care cost trend rate							decreasing
Ultimate health care cost trend rate		4.50%					4.50%
Year that rate reaches ultimate trend rate							2032
Other Postretirement Plans [Member] | Post-Age 65 [Member]
Retirement Benefits Disclosure [Line Items]
Assumed health care cost trend rate		8.04%					8.04%
Direction of change for assumed health care cost trend rate							decreasing
Ultimate health care cost trend rate		4.50%					4.50%
Year that rate reaches ultimate trend rate							2032
Other Postretirement Plans [Member] | Medicare Advantage [Member]
Retirement Benefits Disclosure [Line Items]
Assumed health care cost trend rate		11.25%					11.25%
Direction of change for assumed health care cost trend rate							decreasing
Ultimate health care cost trend rate		4.50%					4.50%
Year that rate reaches ultimate trend rate							2032
Private investment fund [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Percentage of total plan assets		17.00%			16.00%		17.00%	16.00%
Single Equity Concentration [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator							10.00%	10.00%
Value of investments		$ 1,011,100,000			$ 842,600,000		$ 1,011,100,000	$ 842,600,000
Percentage of total plan assets		34.00%			33.00%		34.00%	33.00%
Single Equity Concentration [Member] | Equity securities [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Number of investments the company's pension plan held which individually exceed 10% of total plan assets | investment		1			1		1	1
Single Equity Concentration [Member] | Private investment fund [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Number of investments the company's pension plan held which individually exceed 10% of total plan assets | investment		1			1		1	1
Defined Benefit Plan Assets Total [Member] | Concentration In Single Entity, Type Of Industry, Foreign Country Or Individual Fund [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Minimum percentage of plan assets considered as significant concentrations in pension plans							10.00%
Separation Incentive Program [Member] | Pension Plans [Member] | Leaf Group and Code3
Retirement Benefits Disclosure [Line Items]
Special separation benefit expense				$ 4,100,000
Separation Incentive Program [Member] | Pension Plans [Member] | Kaplan, Graham Media Group, Leaf Group, Code3 and Pinna
Retirement Benefits Disclosure [Line Items]
Special separation benefit expense			$ 5,500,000
Separation Incentive Program [Member] | Pension Plans [Member] | Group Dekko [Member]
Retirement Benefits Disclosure [Line Items]
Special separation benefit expense						$ 1,100,000
Television Broadcasting [Member] | Separation Incentive Program [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Special separation benefit expense		$ 200,000
Graham Healthcare Group [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Service cost							$ 13,500,000	$ 10,500,000
Education [Member] | Separation Incentive Program [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Special separation benefit expense					$ 3,600,000
Maximum [Member] | Graham Healthcare Group [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Pension credit per employee	$ 50,000
Investment Manager 1 [Member] | Alphabet And Berkshire Hathaway Common Stock [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator							15.00%
Investment Manager 1 [Member] | Maximum [Member] | Foreign Investments [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets		35.00%					35.00%
Investment Manager 2 [Member] | Berkshire Hathaway Common Stock [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator							20.00%
Investment Manager 2 [Member] | Maximum [Member] | Foreign Investments [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets		15.00%					15.00%
Investment Manager 2 [Member] | Minimum [Member] | Fixed income securities [Member] | Pension Plans [Member]
Retirement Benefits Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets		10.00%					10.00%