v3.26.1
Segment Information - Schedule of Segments Pre and After Tax Income, Significant Expenses and Reconciliations (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total segment revenues $ 1,387,035 $ 1,326,745 $ 2,745,960 $ 2,611,931
(Favorable)/unfavorable prior year casualty reserve development     (27,219) 34,648
Current year casualty loss costs     1,658,989 1,535,575
Net 816,266 823,898 1,631,770 1,570,223
Net income (loss) 162,845 108,905 287,047 247,791
Income tax (expense) benefit (33,460) (22,962) (59,986) (51,952)
Net income (loss) 129,385 85,943 227,061 195,839
Interest expense (13,215) (13,256) (26,436) (22,829)
Corporate expenses (10,467) (7,556) (28,371) (25,654)
Preferred stock dividends (2,300) (2,300) (4,600) (4,600)
Net income (loss) available to common stockholders, basic 127,085 83,643 222,461 191,239
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       50,000
Insurance Operations        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net 816,266 823,898 1,631,770 1,570,223
Dividends to policyholders 490 1,151 1,190 2,134
Standard Commercial Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net 652,249 666,766 1,309,083 1,248,432
Dividends to policyholders 490 1,151 1,190 2,134
Standard Personal Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net 69,004 69,977 138,303 146,646
E&S Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net 95,013 87,155 184,384 175,145
Operating Segments        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total segment revenues 1,377,646 1,320,197 2,728,924 2,599,874
Net catastrophe losses 68,539 79,932 143,889 123,289
Non-catastrophe property loss and loss expense 170,498 173,239 348,557 351,935
Net 816,266 823,898 1,631,770 1,570,223
Commissions to distribution partners 212,954 213,697 429,409 423,926
Salaries and employee benefits 101,373 94,970 202,807 191,005
Other segment expenses 60,036 56,764 121,894 116,312
Total net underwriting expenses incurred 374,363 365,431 754,110 731,243
Dividends to policyholders 490 1,151 1,190 2,134
Net income (loss) 186,527 129,717 341,854 296,274
Income tax (expense) benefit (38,597) (26,914) (70,633) (61,616)
Net income (loss) 147,930 102,803 271,221 234,658
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 45,000 0 50,000
Current year casualty loss costs 577,229 525,727 1,139,324 1,044,999
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 1,215,508 1,188,057 2,432,704 2,346,814
Net catastrophe losses 68,539 79,932 143,889 123,289
Non-catastrophe property loss and loss expense 170,498 173,239 348,557 351,935
Net 816,266 823,898 1,631,770 1,570,223
Commissions to distribution partners 212,954 213,697 429,409 423,926
Salaries and employee benefits 101,373 94,970 202,807 191,005
Other segment expenses 60,036 56,764 121,894 116,312
Total net underwriting expenses incurred 374,363 365,431 754,110 731,243
Dividends to policyholders 490 1,151 1,190 2,134
Net income (loss) 24,389 (2,423) 45,634 43,214
Income tax (expense) benefit (5,122) 509 (9,583) (9,075)
Net income (loss) 19,267 (1,914) 36,051 34,139
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 45,000 0 50,000
Current year casualty loss costs 577,229 525,727 1,139,324 1,044,999
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 962,044 937,635 1,927,803 1,849,845
Net catastrophe losses 48,666 50,881 105,849 70,692
Non-catastrophe property loss and loss expense 122,894 131,883 250,668 260,675
Net 652,249 666,766 1,309,083 1,248,432
Commissions to distribution partners 171,877 173,406 347,404 343,577
Salaries and employee benefits 81,857 78,667 166,779 158,455
Other segment expenses 48,366 43,789 97,944 90,473
Total net underwriting expenses incurred 302,100 295,862 612,127 592,505
Dividends to policyholders 490 1,151 1,190 2,134
Net income (loss) 7,205 (26,144) 5,403 6,774
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 45,000 0 45,000
Current year casualty loss costs 480,689 439,002 952,566 872,065
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 97,639 102,377 197,667 206,032
Net catastrophe losses 11,937 14,591 25,137 21,704
Non-catastrophe property loss and loss expense 31,195 28,271 60,450 64,759
Net 69,004 69,977 138,303 146,646
Commissions to distribution partners 5,465 6,522 11,321 13,874
Salaries and employee benefits 10,296 8,805 19,392 17,443
Other segment expenses 8,446 8,523 17,065 17,482
Total net underwriting expenses incurred 24,207 23,850 47,778 48,799
Dividends to policyholders 0 0 0 0
Net income (loss) 4,428 8,550 11,586 10,587
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 0 0 5,000
Current year casualty loss costs 25,872 27,115 52,716 55,183
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 155,825 148,045 307,234 290,937
Net catastrophe losses 7,936 14,460 12,903 30,893
Non-catastrophe property loss and loss expense 16,409 13,085 37,439 26,501
Net 95,013 87,155 184,384 175,145
Commissions to distribution partners 35,612 33,769 70,684 66,475
Salaries and employee benefits 9,220 7,498 16,636 15,107
Other segment expenses 3,224 4,452 6,885 8,357
Total net underwriting expenses incurred 48,056 45,719 94,205 89,939
Dividends to policyholders 0 0 0 0
Net income (loss) 12,756 15,171 28,645 25,853
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 70,668 59,610 134,042 117,751
Operating Segments | Investments        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total segment revenues 162,138 132,140 296,220 253,060
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) 162,138 132,140 296,220 253,060
Income tax (expense) benefit (33,475) (27,423) (61,050) (52,541)
Net income (loss) 128,663 104,717 235,170 200,519
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 $ 5,136 $ 3,952 $ 10,647 $ 9,664