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Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
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Jun. 30, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022
Effects of Reinsurance [Line Items] |  |  |  |
Direct premiums written | $ 3,109,910 | $ 2,773,449 | $ 5,652,484 | $ 5,293,678
Direct premiums earned | 2,674,886 | 2,379,794 | 5,287,875 | 4,777,369
Assumed premiums written | 678,275 | 506,600 | 1,571,713 | 1,384,658
Assumed premiums earned | 583,479 | 447,992 | 1,056,915 | 899,919
Ceded premiums written | (1,580,492) | (1,180,254) | (2,798,726) | (2,413,807)
Ceded premiums earned | (1,227,222) | (994,682) | (2,345,943) | (2,084,414)
Net premiums written | 2,207,693 | 2,099,795 | 4,425,471 | 4,264,529
Net premiums earned | 2,031,143 | 1,833,104 | 3,998,847 | 3,592,874
Underwriting [Member] |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Direct premiums written | 2,285,739 | 2,108,859 | 4,255,102 | 3,932,114
Direct premiums earned | 2,025,324 | 1,772,695 | 3,997,200 | 3,470,665
Assumed premiums written | 448,837 | 420,026 | 1,137,732 | 1,116,386
Assumed premiums earned | 424,404 | 382,876 | 823,123 | 785,943
Ceded premiums written | (526,562) | (427,959) | (966,591) | (781,341)
Ceded premiums earned | (418,264) | (321,438) | (820,704) | (661,367)
Net premiums written | 2,208,014 | 2,100,926 | 4,426,243 | 4,267,159
Net premiums earned | 2,031,464 | 1,834,133 | 3,999,619 | 3,595,241
Program services and other fronting [Member] |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Direct premiums written | 824,171 | 664,590 | 1,397,382 | 1,361,564
Direct premiums earned | 649,562 | 607,099 | 1,290,675 | 1,306,704
Assumed premiums written | 229,438 | 86,574 | 433,981 | 268,272
Assumed premiums earned | 159,075 | 65,116 | 233,792 | 113,976
Ceded premiums written | (1,053,930) | (752,295) | (1,832,135) | (1,632,466)
Ceded premiums earned | (808,958) | (673,244) | (1,525,239) | (1,423,047)
Net premiums written | (321) | (1,131) | (772) | (2,630)
Net premiums earned | $ (321) | $ (1,029) | $ (772) | $ (2,367)

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