v3.26.3
Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting Information [Line Items]        
Premiums $ 1,611.5 $ 1,550.7 $ 4,779.7 $ 4,604.5
Net investment income 131.9 117.0 378.4 328.6
Fees and other income 6.2 6.3 18.6 18.8
Total operating revenues 1,749.6 1,674.0 5,176.7 4,951.9
Net realized and unrealized investment gains (losses) (14.1) (9.0) (13.6) (29.3)
Total revenues 1,735.5 1,665.0 5,163.1 4,922.6
Current accident year losses and LAE 898.3 893.0 2,674.2 2,640.6
Prior year unfavorable (favorable) development, excluding catastrophes (18.3) (12.1) (64.8) (50.3)
Current year catastrophe losses 121.7 46.2 374.1 267.3
Prior year favorable catastrophe development (12.0)   (73.7) (18.0)
Total losses and LAE 989.7 927.1 2,909.8 2,839.6
Amortization of deferred acquisition costs and other underwriting expenses 502.7 490.3 1,489.8 1,438.4
Underwriting income (loss) 119.1 133.3 380.1 326.5
Other segment items (12.8) (8.9) (30.6) (29.9)
Operating income (loss) before interest expense and income taxes 244.4 247.7 746.5 644.0
Interest on debt (10.0) (11.6) (30.9) (28.7)
Operating income before income taxes 234.4 236.1 715.6 615.3
Income from continuing operations before income taxes 220.3 227.1 702.0 586.0
Core Commercial [Member]        
Segment Reporting Information [Line Items]        
Premiums 582.2 554.2 1,725.3 1,649.5
Net investment income 60.5 51.8 174.1 147.5
Fees and other income 1.3 1.3 4.0 3.9
Total operating revenues 644.0 607.3 1,903.4 1,800.9
Current accident year losses and LAE 352.4 335.6 1,024.2 983.4
Prior year unfavorable (favorable) development, excluding catastrophes (3.0) (1.2) (5.2) (5.5)
Current year catastrophe losses 49.9 17.8 136.2 97.5
Prior year favorable catastrophe development (4.6)   (34.1) (11.0)
Total losses and LAE 394.7 352.2 1,121.1 1,064.4
Amortization of deferred acquisition costs and other underwriting expenses 191.8 187.8 567.5 553.7
Underwriting income (loss) (4.3) 14.2 36.7 31.4
Other segment items (2.0) (2.2) (7.0) (7.0)
Operating income (loss) before interest expense and income taxes 55.5 65.1 207.8 175.8
Specialty [Member]        
Segment Reporting Information [Line Items]        
Premiums 369.5 353.9 1,095.2 1,049.4
Net investment income 30.5 26.1 86.4 74.7
Fees and other income 1.3 1.3 3.7 3.7
Total operating revenues 401.3 381.3 1,185.3 1,127.8
Current accident year losses and LAE 176.5 172.7 541.8 520.3
Prior year unfavorable (favorable) development, excluding catastrophes (14.8) (10.0) (39.8) (38.4)
Current year catastrophe losses 14.8 6.0 43.4 39.3
Prior year favorable catastrophe development (2.4)   (11.4) (4.0)
Total losses and LAE 174.1 168.7 534.0 517.2
Amortization of deferred acquisition costs and other underwriting expenses 136.8 132.4 404.3 390.4
Underwriting income (loss) 58.6 52.8 156.9 141.8
Other segment items (2.3) (2.0) (6.5) (6.2)
Operating income (loss) before interest expense and income taxes 88.1 78.2 240.5 214.0
Personal Lines [Member]        
Segment Reporting Information [Line Items]        
Premiums 659.8 642.6 1,959.2 1,905.6
Net investment income 36.8 33.0 105.3 93.7
Fees and other income 3.5 3.7 10.8 11.1
Total operating revenues 700.1 679.3 2,075.3 2,010.4
Current accident year losses and LAE 369.4 384.7 1,108.2 1,136.9
Prior year unfavorable (favorable) development, excluding catastrophes (0.7) (0.9) (20.0) (6.3)
Current year catastrophe losses 57.0 22.4 194.5 130.5
Prior year favorable catastrophe development (5.0)   (28.2) (3.0)
Total losses and LAE 420.7 406.2 1,254.5 1,258.1
Amortization of deferred acquisition costs and other underwriting expenses 174.1 170.1 518.0 494.3
Underwriting income (loss) 65.0 66.3 186.7 153.2
Other segment items (1.7) (1.9) (5.1) (5.3)
Operating income (loss) before interest expense and income taxes 103.6 101.1 297.7 252.7
Other [Member]        
Segment Reporting Information [Line Items]        
Premiums 0.0 0.0 0.0 0.0
Net investment income 4.1 6.1 12.6 12.7
Fees and other income 0.1 0.0 0.1 0.1
Total operating revenues 4.2 6.1 12.7 12.8
Current accident year losses and LAE 0.0 0.0 0.0 0.0
Prior year unfavorable (favorable) development, excluding catastrophes 0.2 (0.0) 0.2 (0.1)
Current year catastrophe losses 0.0 0.0 0.0 0.0
Prior year favorable catastrophe development 0.0   0.0 0.0
Total losses and LAE 0.2 (0.0) 0.2 (0.1)
Amortization of deferred acquisition costs and other underwriting expenses 0.0 0.0 0.0 0.0
Underwriting income (loss) (0.2) 0.0 (0.2) 0.1
Other segment items (6.8) (2.8) (12.0) (11.4)
Operating income (loss) before interest expense and income taxes $ (2.8) $ (3.3) $ 0.5 $ 1.5