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Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Effects of Reinsurance [Line Items] |  |  |
Direct premiums written | $ 11,306,483 | $ 10,729,950 | $ 9,508,184
Direct premiums earned | 10,999,889 | 10,068,570 | 8,729,068
Assumed premiums written | 2,695,754 | 2,471,732 | 1,930,498
Assumed premiums earned | 2,554,030 | 2,246,805 | 1,744,346
Ceded premiums written | (5,604,662) | (4,998,292) | (4,318,951)
Ceded premiums earned | (5,258,440) | (4,727,583) | (3,970,385)
Net premiums written | 8,397,575 | 8,203,390 | 7,119,731
Net premiums earned | 8,295,479 | 7,587,792 | 6,503,029
Underwriting [Member] |  |  |
Effects of Reinsurance [Line Items] |  |  |
Direct premiums written | 8,657,266 | 8,085,812 | 6,863,229
Direct premiums earned | 8,363,710 | 7,379,766 | 6,275,078
Assumed premiums written | 1,620,366 | 1,761,726 | 1,622,700
Assumed premiums earned | 1,616,630 | 1,589,920 | 1,482,755
Ceded premiums written | (1,878,844) | (1,640,165) | (1,360,763)
Ceded premiums earned | (1,683,648) | (1,378,191) | (1,250,392)
Net premiums written | 8,398,788 | 8,207,373 | 7,125,166
Net premiums earned | 8,296,692 | 7,591,495 | 6,507,441
Program services and other fronting [Member] |  |  |
Effects of Reinsurance [Line Items] |  |  |
Direct premiums written | 2,649,217 | 2,644,138 | 2,644,955
Direct premiums earned | 2,636,179 | 2,688,804 | 2,453,990
Assumed premiums written | 1,075,388 | 710,006 | 307,798
Assumed premiums earned | 937,400 | 656,885 | 261,591
Ceded premiums written | (3,725,818) | (3,358,127) | (2,958,188)
Ceded premiums earned | (3,574,792) | (3,349,392) | (2,719,993)
Net premiums written | (1,213) | (3,983) | (5,435)
Net premiums earned | $ (1,213) | $ (3,703) | $ (4,412)

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