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Segment Information - Schedule of Segments Pre and After Tax Income, Significant Expenses and Reconciliations (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Revenue from External Customer [Line Items] |  |  |
Total revenues | $ 4,861,664 | $ 4,232,106 | $ 3,558,062
(Favorable)/unfavorable prior year casualty reserve development | 285,271 | 9,954 | (78,890)
Current year casualty loss costs | 2,879,213 | 2,474,331 | 2,190,668
Net | 3,164,484 | 2,484,285 | 2,111,778
Income before federal income tax | 258,034 | 458,412 | 280,186
Total federal income tax expense | (51,022) | (93,174) | (55,300)
Net income | 207,012 | 365,238 | 224,886
Interest expense | (28,878) | (28,799) | (28,847)
Corporate expenses | (34,602) | (30,686) | (31,116)
Preferred stock dividends | (9,200) | (9,200) | (9,200)
Net income available to common stockholders | 197,812 | 356,038 | 215,686
Total Insurance Operations |  |  |
Revenue from External Customer [Line Items] |  |  |
Net | 3,164,484 | 2,484,285 | 2,111,778
Dividends to policyholders | 6,504 | 6,755 | 4,858
Standard Commercial Lines |  |  |
Revenue from External Customer [Line Items] |  |  |
Total revenues | 3,475,950 | 3,088,952 | 2,749,338
Net | 2,501,615 | 1,919,204 | 1,683,988
Dividends to policyholders | 6,504 | 6,755 | 4,858
Standard Personal Lines |  |  |
Revenue from External Customer [Line Items] |  |  |
Total revenues | 427,483 | 367,414 | 301,221
Net | 364,601 | 353,185 | 231,113
E&S Lines |  |  |
Revenue from External Customer [Line Items] |  |  |
Total revenues | 504,129 | 390,642 | 334,156
Net | 298,268 | 211,896 | 196,677
Operating Segments |  |  |
Revenue from External Customer [Line Items] |  |  |
Total revenues | 4,861,664 | 4,232,106 | 3,558,062
Net catastrophe losses | 284,503 | 244,510 | 145,926
Non-catastrophe property loss and loss expense | 681,607 | 650,384 | 617,870
Net | 3,164,484 | 2,484,285 | 2,111,778
Commissions to distribution partners | 780,799 | 680,185 | 604,200
Salaries and employee benefits | 353,451 | 328,005 | 297,512
Other segment expenses | 234,912 | 214,979 | 199,565
Total net underwriting expenses incurred | 1,369,162 | 1,223,169 | 1,101,277
Dividends to policyholders | 6,504 | 6,755 | 4,858
Income before federal income tax | 321,514 | 517,897 | 340,149
Total federal income tax expense | (65,972) | (106,257) | (66,874)
Net income | 255,542 | 411,640 | 273,275
Operating Segments | Casualty lines |  |  |
Revenue from External Customer [Line Items] |  |  |
(Favorable)/unfavorable prior year casualty reserve development | 311,000 | (6,500) | (86,000)
Current year casualty loss costs | 1,887,374 | 1,595,891 | 1,433,982
Operating Segments | Total Insurance Operations |  |  |
Revenue from External Customer [Line Items] |  |  |
Total revenues | 4,407,562 | 3,847,008 | 3,384,715
Net catastrophe losses | 284,503 | 244,510 | 145,926
Non-catastrophe property loss and loss expense | 681,607 | 650,384 | 617,870
Net | 3,164,484 | 2,484,285 | 2,111,778
Commissions to distribution partners | 780,799 | 680,185 | 604,200
Salaries and employee benefits | 353,451 | 328,005 | 297,512
Other segment expenses | 234,912 | 214,979 | 199,565
Total net underwriting expenses incurred | 1,369,162 | 1,223,169 | 1,101,277
Dividends to policyholders | 6,504 | 6,755 | 4,858
Income before federal income tax | (132,588) | 132,799 | 166,802
Total federal income tax expense | 27,843 | (27,888) | (35,028)
Net income | (104,745) | 104,911 | 131,774
Operating Segments | Total Insurance Operations | Casualty lines |  |  |
Revenue from External Customer [Line Items] |  |  |
(Favorable)/unfavorable prior year casualty reserve development | 311,000 | (6,500) | (86,000)
Current year casualty loss costs | 1,887,374 | 1,595,891 | 1,433,982
Operating Segments | Standard Commercial Lines |  |  |
Revenue from External Customer [Line Items] |  |  |
Total revenues | 3,475,950 | 3,088,952 | 2,749,338
Net catastrophe losses | 181,546 | 150,518 | 95,600
Non-catastrophe property loss and loss expense | 459,537 | 461,593 | 461,119
Net | 2,501,615 | 1,919,204 | 1,683,988
Commissions to distribution partners | 639,180 | 571,078 | 515,938
Salaries and employee benefits | 290,880 | 267,375 | 245,339
Other segment expenses | 182,754 | 167,234 | 155,519
Total net underwriting expenses incurred | 1,112,814 | 1,005,687 | 916,796
Dividends to policyholders | 6,504 | 6,755 | 4,858
Income before federal income tax | (144,983) | 157,306 | 143,696
Net income |  |  |
Operating Segments | Standard Commercial Lines | Casualty lines |  |  |
Revenue from External Customer [Line Items] |  |  |
(Favorable)/unfavorable prior year casualty reserve development | 286,000 | (15,500) | (81,000)
Current year casualty loss costs | 1,574,532 | 1,322,593 | 1,208,269
Operating Segments | Standard Personal Lines |  |  |
Revenue from External Customer [Line Items] |  |  |
Total revenues | 427,483 | 367,414 | 301,221
Net catastrophe losses | 79,965 | 69,315 | 40,759
Non-catastrophe property loss and loss expense | 164,045 | 156,890 | 117,125
Net | 364,601 | 353,185 | 231,113
Commissions to distribution partners | 30,270 | 22,052 | 13,379
Salaries and employee benefits | 37,024 | 38,830 | 34,045
Other segment expenses | 35,073 | 32,610 | 29,877
Total net underwriting expenses incurred | 102,367 | 93,492 | 77,301
Dividends to policyholders | 0 | 0 | 0
Income before federal income tax | (39,485) | (79,263) | (7,193)
Net income |  |  |
Operating Segments | Standard Personal Lines | Casualty lines |  |  |
Revenue from External Customer [Line Items] |  |  |
(Favorable)/unfavorable prior year casualty reserve development | 5,000 | 14,000 | 0
Current year casualty loss costs | 115,591 | 112,980 | 73,229
Operating Segments | E&S Lines |  |  |
Revenue from External Customer [Line Items] |  |  |
Total revenues | 504,129 | 390,642 | 334,156
Net catastrophe losses | 22,992 | 24,677 | 9,567
Non-catastrophe property loss and loss expense | 58,025 | 31,901 | 39,626
Net | 298,268 | 211,896 | 196,677
Commissions to distribution partners | 111,349 | 87,055 | 74,883
Salaries and employee benefits | 25,547 | 21,800 | 18,128
Other segment expenses | 17,085 | 15,135 | 14,169
Total net underwriting expenses incurred | 153,981 | 123,990 | 107,180
Dividends to policyholders | 0 | 0 | 0
Income before federal income tax | 51,880 | 54,756 | 30,299
Net income |  |  |
Operating Segments | E&S Lines | Casualty lines |  |  |
Revenue from External Customer [Line Items] |  |  |
(Favorable)/unfavorable prior year casualty reserve development | 20,000 | (5,000) | (5,000)
Current year casualty loss costs | 197,251 | 160,318 | 152,484
Operating Segments | Investments |  |  |
Revenue from External Customer [Line Items] |  |  |
Total revenues | 454,102 | 385,098 | 173,347
Net catastrophe losses | 0 | 0 | 0
Non-catastrophe property loss and loss expense | 0 | 0 | 0
Net | 0 | 0 | 0
Commissions to distribution partners | 0 | 0 | 0
Salaries and employee benefits | 0 | 0 | 0
Other segment expenses | 0 | 0 | 0
Total net underwriting expenses incurred | 0 | 0 | 0
Dividends to policyholders | 0 | 0 | 0
Income before federal income tax | 454,102 | 385,098 | 173,347
Total federal income tax expense | (93,815) | (78,369) | (31,846)
Net income | 360,287 | 306,729 | 141,501
Operating Segments | Investments | Casualty lines |  |  |
Revenue from External Customer [Line Items] |  |  |
(Favorable)/unfavorable prior year casualty reserve development | 0 | 0 | 0
Current year casualty loss costs | 0 | 0 | 0
Reconciling Item, Corporate |  |  |
Revenue from External Customer [Line Items] |  |  |
Total federal income tax expense | $ 14,950 | $ 13,083 | $ 11,574

X
- Definition Severe loss, as defined by the Insurance Services Office's Property Claims Service (PCS) unit, either natural or man-made, usually involving, but not limited to, many risks from one occurrence such as fire, hurricane, tornado, earthquake, windstorm, explosion, hail, severe winter weather, and terrorism. + References No definition available. + Details
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