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Products, Services and Other Revenues (Schedule Of Revenues From Contracts With Customers By Type) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | $ 1,327,151 | $ 1,278,184 | $ 3,928,111 | $ 3,807,696
Income (loss) from equity method investments | 4,200 | (7,100) | (400) | (36,100)
Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 1,230,327 | 1,197,164 | 3,657,610 | 3,499,966
Products [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 585,707 | 575,277 | 1,921,773 | 1,809,276
Services [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 600,416 | 606,246 | 1,659,729 | 1,635,493
Investment management [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 44,204 | 15,641 | 76,108 | 55,197
Program services and other fronting [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 39,499 | 40,033 | 105,952 | 103,096
Disposition gain |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues |  |  | 16,923 | 107,293
Equity method investments income (loss) [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Income (loss) from equity method investments | 4,243 | (7,101) | (398) | (36,102)
Other [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 53,082 | 48,088 | 148,024 | 133,443
Markel Ventures [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 1,245,577 | 1,215,746 | 3,735,329 | 3,527,429
Markel Ventures [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 1,183,219 | 1,169,994 | 3,573,960 | 3,403,222
Markel Ventures [Member] | Products [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 585,707 | 575,277 | 1,921,773 | 1,809,276
Markel Ventures [Member] | Services [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 597,512 | 594,717 | 1,652,187 | 1,593,946
Markel Ventures [Member] | Investment management [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 0 | 0 | 0 | 0
Markel Ventures [Member] | Program services and other fronting [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 0 | 0 | 0 | 0
Markel Ventures [Member] | Disposition gain |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues |  |  | 0 | 0
Markel Ventures [Member] | Equity method investments income (loss) [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Income (loss) from equity method investments | (21) | (546) | 1,087 | 1,537
Markel Ventures [Member] | Other [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 62,379 | 46,298 | 160,282 | 122,670
Other [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 81,574 | 62,438 | 192,782 | 280,267
Other [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 47,108 | 27,170 | 83,650 | 96,744
Other [Member] | Products [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 0 | 0 | 0 | 0
Other [Member] | Services [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 2,904 | 11,529 | 7,542 | 41,547
Other [Member] | Investment management [Member] | Revenue from contracts with customers [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 44,204 | 15,641 | 76,108 | 55,197
Other [Member] | Program services and other fronting [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | 39,499 | 40,033 | 105,952 | 103,096
Other [Member] | Disposition gain |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues |  |  | 16,923 | 107,293
Other [Member] | Equity method investments income (loss) [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Income (loss) from equity method investments | 4,264 | (6,555) | (1,485) | (37,639)
Other [Member] | Other [Member] |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Other revenues | $ (9,297) | $ 1,790 | $ (12,258) | $ 10,773

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