Pension and Postretirement Plans (Narrative) (Details) - Defined Benefit Pension Plan [Member] $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2024 USD ($) investment company	Mar. 31, 2024 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Jun. 30, 2024 USD ($) investment company	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($) investment
Defined Benefit Plan Disclosure [Line Items]
Early retirement programs and special separation benefit expense	$ 16,410		$ 5,517		$ 16,828	$ 9,646
Percent of plan assets managed internally by the company					48.00%
Percent of plan assets managed by investment companies					52.00%
Number of investment companies actively managing plan assets | company	2				2
Percentage of total plan assets	100.00%				100.00%		100.00%
Markel Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Value of investments	$ 91,400				$ 91,400		$ 82,400
Berkshire Hathaway Common Stock [Member] | Investment Manager 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator					20.00%
Alphabet and Berkshire Hathaway Common Stock [Member] | Investment Manager 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator					15.00%
Foreign Investments [Member] | Maximum [Member] | Investment Manager 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	35.00%				35.00%
Foreign Investments [Member] | Maximum [Member] | Investment Manager 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	15.00%				15.00%
Fixed income securities [Member] | Minimum [Member] | Investment Manager 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	5.00%				5.00%
Private investment fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of total plan assets	15.00%				15.00%		17.00%
Single Equity Concentration [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator					10.00%
Value of investments	$ 1,602,300				$ 1,602,300		$ 1,011,100
Percentage of total plan assets	49.00%				49.00%		34.00%
Single Equity Concentration [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of investments the company's pension plan held which individually exceed 10% of total plan assets | investment	1				1		1
Single Equity Concentration [Member] | Private investment fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of investments the company's pension plan held which individually exceed 10% of total plan assets | investment	1				1		1
Single Equity Concentration [Member] | US Treasury Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of investments the company's pension plan held which individually exceed 10% of total plan assets | investment	1				1
Concentration In Single Entity, Type Of Industry, Foreign Country Or Individual Fund [Member] | Defined Benefit Plan Assets Total [Member]
Defined Benefit Plan Disclosure [Line Items]
Minimum percentage of plan assets considered as significant concentrations in pension plans		10.00%			10.00%
Leaf Group and Code3 [Member] | Separation Incentive Program [Member]
Defined Benefit Plan Disclosure [Line Items]
Early retirement programs and special separation benefit expense				$ 4,100
Kaplan, Graham Media Group, Leaf Group, Code3 and Pinna [Member] | Separation Incentive Program [Member]
Defined Benefit Plan Disclosure [Line Items]
Early retirement programs and special separation benefit expense			$ 5,500
Framebridge [Member] | Separation Incentive Program [Member]
Defined Benefit Plan Disclosure [Line Items]
Early retirement programs and special separation benefit expense		$ 400
Graham Media Group and Corporate [Member] | VoluntaryRetirementIncentiveProgram [Member]
Defined Benefit Plan Disclosure [Line Items]
Early retirement programs and special separation benefit expense	$ 14,800
Framebridge and Code3 [Member] | Separation Incentive Program [Member]
Defined Benefit Plan Disclosure [Line Items]
Early retirement programs and special separation benefit expense	$ 1,600