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Segment Information - Schedule of Combined Ratio (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Revenue from External Customer [Line Items] |  |  |
Net premiums earned | $ 4,376,447 | $ 3,827,606 | $ 3,373,380
Loss and loss expense incurred | 3,164,484 | 2,484,285 | 2,111,778
Total Insurance Operations |  |  |
Revenue from External Customer [Line Items] |  |  |
Net premiums earned | 4,376,447 | 3,827,606 | 3,373,380
Loss and loss expense incurred | 3,164,484 | 2,484,285 | 2,111,778
Net underwriting expenses incurred | 1,338,047 | 1,203,767 | 1,089,942
Dividends to policyholders | 6,504 | 6,755 | 4,858
Underwriting income (loss) | $ (132,588) | $ 132,799 | $ 166,802
Ratio |  |  |
Loss and loss expense incurred (as a percent) | 72.30% | 64.90% | 62.70%
Net underwriting expenses incurred (as a percent) | 30.60% | 31.40% | 32.30%
Dividends to policyholders (as a percent) | 0.10% | 0.20% | 0.10%
Underwriting income (loss) (as a percent) | 103.00% | 96.50% | 95.10%
Standard Commercial Lines |  |  |
Revenue from External Customer [Line Items] |  |  |
Net premiums earned | $ 3,447,556 | $ 3,071,784 | $ 2,739,819
Loss and loss expense incurred | 2,501,615 | 1,919,204 | 1,683,988
Net underwriting expenses incurred | 1,084,420 | 988,519 | 907,277
Dividends to policyholders | 6,504 | 6,755 | 4,858
Underwriting income (loss) | $ (144,983) | $ 157,306 | $ 143,696
Ratio |  |  |
Loss and loss expense incurred (as a percent) | 72.50% | 62.50% | 61.50%
Net underwriting expenses incurred (as a percent) | 31.50% | 32.20% | 33.10%
Dividends to policyholders (as a percent) | 0.20% | 0.20% | 0.20%
Underwriting income (loss) (as a percent) | 104.20% | 94.90% | 94.80%
Standard Personal Lines |  |  |
Revenue from External Customer [Line Items] |  |  |
Net premiums earned | $ 424,917 | $ 365,213 | $ 299,405
Loss and loss expense incurred | 364,601 | 353,185 | 231,113
Net underwriting expenses incurred | 99,801 | 91,291 | 75,485
Underwriting income (loss) | $ (39,485) | $ (79,263) | $ (7,193)
Ratio |  |  |
Loss and loss expense incurred (as a percent) | 85.80% | 96.70% | 77.20%
Net underwriting expenses incurred (as a percent) | 23.50% | 25.00% | 25.20%
Underwriting income (loss) (as a percent) | 109.30% | 121.70% | 102.40%
E&S Lines |  |  |
Revenue from External Customer [Line Items] |  |  |
Net premiums earned | $ 503,974 | $ 390,609 | $ 334,156
Loss and loss expense incurred | 298,268 | 211,896 | 196,677
Net underwriting expenses incurred | 153,826 | 123,957 | 107,180
Underwriting income (loss) | $ 51,880 | $ 54,756 | $ 30,299
Ratio |  |  |
Loss and loss expense incurred (as a percent) | 59.20% | 54.30% | 58.80%
Net underwriting expenses incurred (as a percent) | 30.50% | 31.70% | 32.10%
Underwriting income (loss) (as a percent) | 89.70% | 86.00% | 90.90%

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