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Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred Policy Acquisition Cost | $ 763,486 | $ 676,145 | $ 556,168
Reserve for Losses and Loss Expenses | 17,011,223 | 15,390,888 | 13,784,430
Unearned Premiums | 5,297,654 | 4,847,160 | 4,073,191
Net Premiums Earned | 9,561,429 | 8,106,031 | 6,930,843
Net Investment Income | 779,185 | 671,618 | 583,821
Loss and Loss Expenses | 5,861,750 | 4,953,960 | 4,468,706
Amortization of Deferred Policy Acquisition Cost | 1,038,903 | 961,628 | 904,955
Other Operating Costs and Expenses | 1,922,602 | 1,637,642 | 1,485,437
Net Premiums Written | 10,004,070 | 8,862,867 | 7,262,437
Operating Segments | Insurance |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred Policy Acquisition Cost | 651,257 | 566,718 | 467,871
Reserve for Losses and Loss Expenses | 13,786,112 | 12,379,395 | 10,977,674
Unearned Premiums | 4,779,214 | 4,348,171 | 3,660,758
Net Premiums Earned | 8,369,062 | 7,077,708 | 6,067,669
Net Investment Income | 550,084 | 468,821 | 375,554
Loss and Loss Expenses | 5,130,909 | 4,326,403 | 3,939,759
Amortization of Deferred Policy Acquisition Cost | 935,469 | 830,199 | 734,062
Other Operating Costs and Expenses | 1,430,456 | 1,202,192 | 1,137,002
Net Premiums Written | 8,784,146 | 7,743,814 | 6,347,101
Operating Segments | Reinsurance & Monoline Excess |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred Policy Acquisition Cost | 112,229 | 109,427 | 88,297
Reserve for Losses and Loss Expenses | 3,225,111 | 3,011,493 | 2,806,756
Unearned Premiums | 518,440 | 498,989 | 412,433
Net Premiums Earned | 1,192,367 | 1,028,323 | 863,174
Net Investment Income | 194,272 | 175,324 | 146,029
Loss and Loss Expenses | 730,841 | 627,557 | 528,947
Amortization of Deferred Policy Acquisition Cost | 103,434 | 131,429 | 170,893
Other Operating Costs and Expenses | 235,836 | 174,098 | 103,775
Net Premiums Written | 1,219,924 | 1,119,053 | 915,336
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 | 34,829 | 27,473 | 62,238
Loss and Loss Expenses | 0 | 0 | 0
Amortization of Deferred Policy Acquisition Cost | 0 | 0 | 0
Other Operating Costs and Expenses | 256,310 | 261,352 | 244,660
Net Premiums Written | $ 0 | $ 0 | $ 0

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