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Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred policy acquisition costs | $ 479,304 | $ 424,864 | $ 368,624
Reserve for loss and loss expense | 6,589,801 | 5,336,911 | 5,144,821
Unearned premiums | 2,616,268 | 2,330,656 | 1,992,781
Net premiums earned | 4,376,447 | 3,827,606 | 3,373,380
Net investment income | 454,102 | 385,098 | 173,347
Loss and loss expense incurred | 3,164,484 | 2,484,285 | 2,111,778
Amortization of deferred policy acquisition costs | 922,431 | 796,182 | 705,822
Other operating expenses | 422,120 | 414,340 | 388,978
Net premiums written | 4,630,001 | 4,134,532 | 3,573,590
Other insurance expenses | 453,235 | 433,742 | 400,313
Other income | (31,115) | (19,402) | (11,335)
Standard Commercial Lines |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred policy acquisition costs | 389,992 | 350,198 | 311,535
Reserve for loss and loss expense | 5,388,205 | 4,494,965 | 4,275,002
Unearned premiums | 1,908,949 | 1,722,706 | 1,511,447
Net premiums earned | 3,447,556 | 3,071,784 | 2,739,819
Net investment income | 0 | 0 | 0
Loss and loss expense incurred | 2,501,615 | 1,919,204 | 1,683,988
Amortization of deferred policy acquisition costs | 764,662 | 674,374 | 605,845
Other operating expenses | 326,262 | 320,900 | 306,290
Net premiums written | 3,632,113 | 3,281,319 | 2,901,984
Standard Personal Lines |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred policy acquisition costs | 23,374 | 24,759 | 17,817
Reserve for loss and loss expense | 541,449 | 278,152 | 340,302
Unearned premiums | 433,056 | 399,157 | 322,668
Net premiums earned | 424,917 | 365,213 | 299,405
Net investment income | 0 | 0 | 0
Loss and loss expense incurred | 364,601 | 353,185 | 231,113
Amortization of deferred policy acquisition costs | 46,487 | 36,001 | 27,129
Other operating expenses | 53,314 | 55,290 | 48,356
Net premiums written | 430,725 | 414,585 | 319,059
E&S Lines |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred policy acquisition costs | 65,938 | 49,907 | 39,272
Reserve for loss and loss expense | 660,147 | 563,794 | 529,517
Unearned premiums | 274,263 | 208,793 | 158,666
Net premiums earned | 503,974 | 390,609 | 334,156
Net investment income | 0 | 0 | 0
Loss and loss expense incurred | 298,268 | 211,896 | 196,677
Amortization of deferred policy acquisition costs | 111,282 | 85,807 | 72,848
Other operating expenses | 42,544 | 38,150 | 34,332
Net premiums written | 567,163 | 438,628 | 352,547
Investments Segment |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |
Deferred policy acquisition costs | 0 | 0 | 0
Reserve for loss and loss expense | 0 | 0 | 0
Unearned premiums | 0 | 0 | 0
Net premiums earned | 0 | 0 | 0
Net investment income | 454,102 | 385,098 | 173,347
Loss and loss expense incurred | 0 | 0 | 0
Amortization of deferred policy acquisition costs | 0 | 0 | 0
Other operating expenses | 0 | 0 | 0
Net premiums written | $ 0 | $ 0 | $ 0

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