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Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred Policy Acquisition Cost | $ 861,609 | $ 763,486 | $ 676,145
Reserve for Losses and Loss Expenses | 18,739,652 | 17,011,223 | 15,390,888
Unearned Premiums | 5,922,326 | 5,297,654 | 4,847,160
Net Premiums Earned | 10,400,687 | 9,561,429 | 8,106,031
Net Investment Income | 1,052,835 | 779,185 | 671,618
Loss and Loss Expenses | 6,372,142 | 5,861,750 | 4,953,960
Amortization of Deferred Policy Acquisition Cost | 1,038,975 | 1,038,903 | 961,628
Other Operating Costs and Expenses | 2,324,961 | 1,922,602 | 1,637,642
Net Premiums Written | 10,954,467 | 10,004,070 | 8,862,867
Operating Segments | Insurance |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred Policy Acquisition Cost | 748,134 | 651,257 | 566,718
Reserve for Losses and Loss Expenses | 15,386,761 | 13,786,112 | 12,379,395
Unearned Premiums | 5,367,137 | 4,779,214 | 4,348,171
Net Premiums Earned | 9,130,324 | 8,369,062 | 7,077,708
Net Investment Income | 793,998 | 550,084 | 468,821
Loss and Loss Expenses | 5,689,263 | 5,130,909 | 4,326,403
Amortization of Deferred Policy Acquisition Cost | 931,748 | 935,469 | 830,199
Other Operating Costs and Expenses | 1,699,703 | 1,430,456 | 1,202,192
Net Premiums Written | 9,657,121 | 8,784,146 | 7,743,814
Operating Segments | Reinsurance & Monoline Excess |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred Policy Acquisition Cost | 113,475 | 112,229 | 109,427
Reserve for Losses and Loss Expenses | 3,352,891 | 3,225,111 | 3,011,493
Unearned Premiums | 555,189 | 518,440 | 498,989
Net Premiums Earned | 1,270,363 | 1,192,367 | 1,028,323
Net Investment Income | 211,628 | 194,272 | 175,324
Loss and Loss Expenses | 682,879 | 730,841 | 627,557
Amortization of Deferred Policy Acquisition Cost | 107,227 | 103,434 | 131,429
Other Operating Costs and Expenses | 253,120 | 235,836 | 174,098
Net Premiums Written | 1,297,346 | 1,219,924 | 1,119,053
Corporate, other and eliminations |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred Policy Acquisition Cost | 0 | 0 | 0
Reserve for Losses and Loss Expenses | 0 | 0 | 0
Unearned Premiums | 0 | 0 | 0
Net Premiums Earned | 0 | 0 | 0
Net Investment Income | 47,209 | 34,829 | 27,473
Loss and Loss Expenses | 0 | 0 | 0
Amortization of Deferred Policy Acquisition Cost | 0 | 0 | 0
Other Operating Costs and Expenses | 372,138 | 256,310 | 261,352
Net Premiums Written | $ 0 | $ 0 | $ 0

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