v3.26.3
Segment Information - Schedule of Segments Pre and After Tax Income, Significant Expenses and Reconciliations (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total segment revenues $ 1,326,300 $ 1,360,110 $ 4,072,260 $ 3,972,041
(Favorable)/unfavorable prior year casualty reserve development     (22,545) 70,096
Current year casualty loss costs     2,417,011 2,319,158
Net 762,696 819,031 2,394,466 2,389,254
Net income (loss) 162,088 145,711 449,135 393,502
Income tax (expense) benefit (33,436) (30,371) (93,422) (82,323)
Net income (loss) 128,652 115,340 355,713 311,179
Interest expense (13,215) (13,253) (39,651) (36,082)
Corporate expenses (3,117) (4,446) (31,488) (30,100)
Preferred stock dividends (2,300) (2,300) (6,900) (6,900)
Net income (loss) available to common stockholders, basic 126,352 113,040 348,813 304,279
Casualty lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
(Favorable)/unfavorable prior year casualty reserve development       90,000
Insurance Operations        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net 762,696 819,031 2,394,466 2,389,254
Dividends to policyholders 913 876 2,103 3,010
Standard Commercial Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net 600,255 657,958 1,909,338 1,906,390
Dividends to policyholders 913 876 2,103 3,010
Standard Personal Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net 73,507 89,290 211,810 235,936
E&S Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net 88,934 71,783 273,318 246,928
Operating Segments        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total segment revenues 1,321,226 1,351,390 4,050,150 3,951,264
Net catastrophe losses 64,099 24,899 207,990 148,188
Non-catastrophe property loss and loss expense 154,532 169,630 503,090 521,565
Net 762,696 819,031 2,394,466 2,389,254
Commissions to distribution partners 205,182 212,527 634,590 636,454
Salaries and employee benefits 101,691 96,406 304,499 287,392
Other segment expenses 72,324 59,140 194,218 175,470
Total net underwriting expenses incurred 379,197 368,073 1,133,307 1,099,316
Dividends to policyholders 913 876 2,103 3,010
Net income (loss) 178,420 163,410 520,274 459,684
Income tax (expense) benefit (36,941) (33,887) (107,574) (95,503)
Net income (loss) 141,479 129,523 412,700 364,181
Operating Segments | Casualty lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
(Favorable)/unfavorable prior year casualty reserve development 0 40,000 0 90,000
Current year casualty loss costs 544,065 584,502 1,683,386 1,629,501
Operating Segments | Insurance Operations        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total segment revenues 1,180,191 1,204,678 3,612,895 3,551,492
Net catastrophe losses 64,099 24,899 207,990 148,188
Non-catastrophe property loss and loss expense 154,532 169,630 503,090 521,565
Net 762,696 819,031 2,394,466 2,389,254
Commissions to distribution partners 205,182 212,527 634,590 636,454
Salaries and employee benefits 101,691 96,406 304,499 287,392
Other segment expenses 72,324 59,140 194,218 175,470
Total net underwriting expenses incurred 379,197 368,073 1,133,307 1,099,316
Dividends to policyholders 913 876 2,103 3,010
Net income (loss) 37,385 16,698 83,019 59,912
Income tax (expense) benefit (7,851) (3,507) (17,434) (12,582)
Net income (loss) 29,534 13,191 65,585 47,330
Operating Segments | Insurance Operations | Casualty lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
(Favorable)/unfavorable prior year casualty reserve development 0 40,000 0 90,000
Current year casualty loss costs 544,065 584,502 1,683,386 1,629,501
Operating Segments | Standard Commercial Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total segment revenues 927,104 947,272 2,854,907 2,797,117
Net catastrophe losses 42,412 15,013 148,263 85,705
Non-catastrophe property loss and loss expense 107,142 118,749 357,811 379,423
Net 600,255 657,958 1,909,338 1,906,390
Commissions to distribution partners 164,687 173,294 512,091 516,871
Salaries and employee benefits 83,196 79,150 249,975 237,604
Other segment expenses 58,903 46,170 156,846 136,644
Total net underwriting expenses incurred 306,786 298,614 918,912 891,119
Dividends to policyholders 913 876 2,103 3,010
Net income (loss) 19,150 (10,176) 24,554 (3,402)
Operating Segments | Standard Commercial Lines | Casualty lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
(Favorable)/unfavorable prior year casualty reserve development 0 35,000 0 80,000
Current year casualty loss costs 450,701 489,196 1,403,264 1,361,262
Operating Segments | Standard Personal Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total segment revenues 95,325 101,519 292,992 307,551
Net catastrophe losses 18,342 12,160 43,480 33,864
Non-catastrophe property loss and loss expense 30,683 39,907 91,134 104,667
Net 73,507 89,290 211,810 235,936
Commissions to distribution partners 5,276 4,083 16,597 17,958
Salaries and employee benefits 10,380 9,620 29,772 27,045
Other segment expenses 9,210 8,769 26,275 26,268
Total net underwriting expenses incurred 24,866 22,472 72,644 71,271
Dividends to policyholders 0 0 0 0
Net income (loss) (3,048) (10,243) 8,538 344
Operating Segments | Standard Personal Lines | Casualty lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
(Favorable)/unfavorable prior year casualty reserve development 0 5,000 0 10,000
Current year casualty loss costs 24,482 32,223 77,196 87,405
Operating Segments | E&S Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total segment revenues 157,762 155,887 464,996 446,824
Net catastrophe losses 3,345 (2,274) 16,247 28,619
Non-catastrophe property loss and loss expense 16,707 10,974 54,145 37,475
Net 88,934 71,783 273,318 246,928
Commissions to distribution partners 35,219 35,150 105,902 101,625
Salaries and employee benefits 8,115 7,636 24,752 22,743
Other segment expenses 4,211 4,201 11,097 12,558
Total net underwriting expenses incurred 47,545 46,987 141,751 136,926
Dividends to policyholders 0 0 0 0
Net income (loss) 21,283 37,117 49,927 62,970
Operating Segments | E&S Lines | Casualty lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
(Favorable)/unfavorable prior year casualty reserve development 0 0 0 0
Current year casualty loss costs 68,882 63,083 202,926 180,834
Operating Segments | Investments        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total segment revenues 141,035 146,712 437,255 399,772
Net catastrophe losses 0 0 0 0
Non-catastrophe property loss and loss expense 0 0 0 0
Net 0 0 0 0
Commissions to distribution partners 0 0 0 0
Salaries and employee benefits 0 0 0 0
Other segment expenses 0 0 0 0
Total net underwriting expenses incurred 0 0 0 0
Dividends to policyholders 0 0 0 0
Net income (loss) 141,035 146,712 437,255 399,772
Income tax (expense) benefit (29,090) (30,380) (90,140) (82,921)
Net income (loss) 111,945 116,332 347,115 316,851
Operating Segments | Investments | Casualty lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
(Favorable)/unfavorable prior year casualty reserve development 0 0 0 0
Current year casualty loss costs 0 0 0 0
Reconciling Item, Corporate        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Income tax (expense) benefit $ 3,504 $ 3,516 $ 14,152 $ 13,180