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Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Deferred policy acquisition costs |  | $ 424,864 |  | $ 368,624 |  | $ 326,915 |
Reserve for loss and loss expenses |  | 5,336,911 |  | 5,144,821 |  | 4,580,903 |
Unearned premiums |  | 2,330,656 |  | 1,992,781 |  | 1,803,207 |
Net premiums earned |  | 3,827,606 |  | 3,373,380 |  | 3,017,253 |
Net investment income | [1] | 385,098 |  | 173,347 |  | 344,188 |
Loss and loss expense incurred |  | 2,484,285 |  | 2,111,778 |  | 1,813,984 |
Amortization of deferred policy acquisition costs |  | 796,182 |  | 705,822 |  | 626,469 |
Other operating expenses |  | 414,340 | [2] | 388,978 | [3] | 358,208 | [4]
Net premiums written |  | 4,134,532 |  | 3,573,590 |  | 3,189,713 |
Other insurance expenses |  | 433,742 |  | 400,313 |  | 375,931 |
Other income |  | 19,402 |  | 11,335 |  | 17,723 |
Standard Commercial Lines [Member] |  |  |  |  |  |  |
Deferred policy acquisition costs |  | 350,198 |  | 311,535 |  | 279,850 |
Reserve for loss and loss expenses |  | 4,494,965 |  | 4,275,002 |  | 3,832,151 |
Unearned premiums |  | 1,722,706 |  | 1,511,447 |  | 1,346,809 |
Net premiums earned |  | 3,071,784 |  | 2,739,819 |  | 2,443,885 |
Net investment income |  | 0 |  | 0 |  | 0 |
Loss and loss expense incurred |  | 1,919,204 |  | 1,683,988 |  | 1,426,768 |
Amortization of deferred policy acquisition costs |  | 674,374 |  | 605,845 |  | 539,606 |
Other operating expenses |  | 320,900 |  | 306,290 |  | 278,915 |
Net premiums written |  | 3,281,319 |  | 2,901,984 |  | 2,593,018 |
E&S Lines [Member] |  |  |  |  |  |  |
Deferred policy acquisition costs |  | 49,907 |  | 39,272 |  | 34,154 |
Reserve for loss and loss expenses |  | 563,794 |  | 529,517 |  | 478,686 |
Unearned premiums |  | 208,793 |  | 158,666 |  | 139,122 |
Net premiums earned |  | 390,609 |  | 334,156 |  | 279,809 |
Net investment income |  | 0 |  | 0 |  | 0 |
Loss and loss expense incurred |  | 211,896 |  | 196,677 |  | 175,100 |
Amortization of deferred policy acquisition costs |  | 85,807 |  | 72,848 |  | 60,945 |
Other operating expenses |  | 38,150 |  | 34,332 |  | 27,734 |
Net premiums written |  | 438,628 |  | 352,547 |  | 304,430 |
Miscellaneous Income [Member] |  |  |  |  |  |  |
Other income |  | 19,402 |  | 11,335 |  | 17,723 |
Standard Personal Lines |  |  |  |  |  |  |
Deferred policy acquisition costs |  | 24,759 |  | 17,817 |  | 12,911 |
Reserve for loss and loss expenses |  | 278,152 |  | 340,302 |  | 270,066 |
Unearned premiums |  | 399,157 |  | 322,668 |  | 317,276 |
Net premiums earned |  | 365,213 |  | 299,405 |  | 293,559 |
Net investment income |  | 0 |  | 0 |  | 0 |
Loss and loss expense incurred |  | 353,185 |  | 231,113 |  | 212,116 |
Amortization of deferred policy acquisition costs |  | 36,001 |  | 27,129 |  | 25,918 |
Other operating expenses |  | 55,290 |  | 48,356 |  | 51,559 |
Net premiums written |  | 414,585 |  | 319,059 |  | 292,265 |
Investments SIGI |  |  |  |  |  |  |
Deferred policy acquisition costs |  | 0 |  | 0 |  | 0 |
Reserve for loss and loss expenses |  | 0 |  | 0 |  | 0 |
Unearned premiums |  | 0 |  | 0 |  | 0 |
Net premiums earned |  | 0 |  | 0 |  | 0 |
Net investment income | [1] | 385,098 |  | 173,347 |  | 344,188 |
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 |
[1] | Includes “Net investment income earned” and “Net realized and unrealized investment gains (losses)” on the Consolidated Statements of Income.
[2] | “Other operating expenses” of $414,340 reconciles to the Consolidated Statements of Income
[3] | “Other operating expenses” of $388,978 reconciles to the Consolidated Statements of Income
[4] | “Other operating expenses” of $358,208 reconciles to the Consolidated Statements of Income

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