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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, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023
Effects of Reinsurance [Line Items] |  |  |  |
Direct premiums written | $ 3,299,676 | $ 3,109,910 | $ 6,152,101 | $ 5,652,484
Direct premiums earned | 2,961,150 | 2,674,886 | 5,863,070 | 5,287,875
Assumed premiums written | 1,412,454 | 678,275 | 2,500,428 | 1,571,713
Assumed premiums earned | 655,332 | 583,479 | 1,171,545 | 1,056,915
Ceded premiums written | (2,433,351) | (1,580,492) | (4,141,713) | (2,798,726)
Ceded premiums earned | (1,539,383) | (1,227,222) | (2,829,889) | (2,345,943)
Net premiums written | 2,278,779 | 2,207,693 | 4,510,816 | 4,425,471
Net premiums earned | 2,077,099 | 2,031,143 | 4,204,726 | 3,998,847
Underwriting [Member] |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Direct premiums written | 2,356,536 | 2,285,739 | 4,464,003 | 4,255,102
Direct premiums earned | 2,149,773 | 2,025,324 | 4,348,011 | 3,997,200
Assumed premiums written | 564,851 | 448,837 | 1,233,550 | 1,137,732
Assumed premiums earned | 414,488 | 424,404 | 807,430 | 823,123
Ceded premiums written | (642,448) | (526,562) | (1,186,404) | (966,591)
Ceded premiums earned | (487,002) | (418,264) | (950,382) | (820,704)
Net premiums written | 2,278,939 | 2,208,014 | 4,511,149 | 4,426,243
Net premiums earned | 2,077,259 | 2,031,464 | 4,205,059 | 3,999,619
Program services and other fronting [Member] |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Direct premiums written | 943,140 | 824,171 | 1,688,098 | 1,397,382
Direct premiums earned | 811,377 | 649,562 | 1,515,059 | 1,290,675
Assumed premiums written | 847,603 | 229,438 | 1,266,878 | 433,981
Assumed premiums earned | 240,844 | 159,075 | 364,115 | 233,792
Ceded premiums written | (1,790,903) | (1,053,930) | (2,955,309) | (1,832,135)
Ceded premiums earned | (1,052,381) | (808,958) | (1,879,507) | (1,525,239)
Net premiums written | (160) | (321) | (333) | (772)
Net premiums earned | $ (160) | $ (321) | $ (333) | $ (772)

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