v3.26.1
Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting Information [Line Items]        
Premiums $ 1,597.6 $ 1,545.3 $ 3,168.2 $ 3,053.8
Net investment income 119.6 105.5 246.5 211.6
Fees and other income 6.2 6.1 12.4 12.5
Total operating revenues 1,723.4 1,656.9 3,427.1 3,277.9
Net realized and unrealized investment gains (losses) 2.8 (2.5) 0.5 (20.3)
Total revenues 1,726.2 1,654.4 3,427.6 3,257.6
Current accident year losses and LAE 892.2 867.9 1,775.9 1,747.6
Prior year favorable development, excluding catastrophes (21.5) (18.2) (46.5) (38.2)
Current year catastrophe losses 104.7 113.5 252.4 221.1
Prior year favorable catastrophe development (12.9) (6.0) (61.7) (18.0)
Total losses and LAE 962.5 957.2 1,920.1 1,912.5
Amortization of deferred acquisition costs and other underwriting expenses 500.3 477.8 987.1 948.1
Underwriting income 134.8 110.3 261.0 193.2
Other segment items (8.7) (12.0) (17.8) (21.0)
Operating income (loss) before interest expense and income taxes 251.9 209.9 502.1 396.3
Interest on debt (10.1) (8.6) (20.9) (17.1)
Operating income before income taxes 241.8 201.3 481.2 379.2
Income from continuing operations before income taxes 244.6 198.8 481.7 358.9
Core Commercial [Member]        
Segment Reporting Information [Line Items]        
Premiums 579.3 554.3 1,143.1 1,095.3
Net investment income 55.3 47.7 113.6 95.7
Fees and other income 1.3 1.3 2.7 2.6
Total operating revenues 635.9 603.3 1,259.4 1,193.6
Current accident year losses and LAE 340.6 313.6 671.8 647.8
Prior year favorable development, excluding catastrophes (0.6) (3.0) (2.2) (4.3)
Current year catastrophe losses 34.2 25.2 86.3 79.7
Prior year favorable catastrophe development (7.8) (2.5) (29.5) (11.0)
Total losses and LAE 366.4 333.3 726.4 712.2
Amortization of deferred acquisition costs and other underwriting expenses 189.7 183.8 375.7 365.9
Underwriting income 23.2 37.2 41.0 17.2
Other segment items (2.3) (2.3) (5.0) (4.8)
Operating income (loss) before interest expense and income taxes 77.5 83.9 152.3 110.7
Specialty [Member]        
Segment Reporting Information [Line Items]        
Premiums 365.8 355.9 725.7 695.5
Net investment income 27.2 24.3 55.9 48.6
Fees and other income 1.2 1.1 2.4 2.4
Total operating revenues 394.2 381.3 784.0 746.5
Current accident year losses and LAE 188.5 174.3 365.3 347.6
Prior year favorable development, excluding catastrophes (10.8) (12.5) (25.0) (28.4)
Current year catastrophe losses 13.2 16.1 28.6 33.3
Prior year favorable catastrophe development (3.2) (1.5) (9.0) (4.0)
Total losses and LAE 187.7 176.4 359.9 348.5
Amortization of deferred acquisition costs and other underwriting expenses 135.9 131.7 267.5 258.0
Underwriting income 42.2 47.8 98.3 89.0
Other segment items (2.2) (2.0) (4.2) (4.2)
Operating income (loss) before interest expense and income taxes 68.4 71.2 152.4 135.8
Personal Lines [Member]        
Segment Reporting Information [Line Items]        
Premiums 652.5 635.1 1,299.4 1,263.0
Net investment income 33.3 30.2 68.5 60.7
Fees and other income 3.7 3.7 7.3 7.4
Total operating revenues 689.5 669.0 1,375.2 1,331.1
Current accident year losses and LAE 363.1 380.0 738.8 752.2
Prior year favorable development, excluding catastrophes (10.1) (2.6) (19.3) (5.4)
Current year catastrophe losses 57.3 72.2 137.5 108.1
Prior year favorable catastrophe development (1.9) (2.0) (23.2) (3.0)
Total losses and LAE 408.4 447.6 833.8 851.9
Amortization of deferred acquisition costs and other underwriting expenses 174.7 162.3 343.9 324.2
Underwriting income 69.4 25.2 121.7 86.9
Other segment items (1.5) (1.7) (3.4) (3.4)
Operating income (loss) before interest expense and income taxes 104.9 57.4 194.1 151.6
Other [Member]        
Segment Reporting Information [Line Items]        
Premiums 0.0 0.0 0.0 0.0
Net investment income 3.8 3.3 8.5 6.6
Fees and other income 0.0 0.0 0.0 0.1
Total operating revenues 3.8 3.3 8.5 6.7
Current accident year losses and LAE 0.0 0.0 0.0 0.0
Prior year favorable development, excluding catastrophes 0.0 (0.1) 0.0 (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 0.0
Total losses and LAE 0.0 (0.1)   (0.1)
Amortization of deferred acquisition costs and other underwriting expenses 0.0 0.0 0.0 0.0
Underwriting income 0.0 0.1 0.0 0.1
Other segment items (2.7) (6.0) (5.2) (8.6)
Operating income (loss) before interest expense and income taxes $ 1.1 $ (2.6) $ 3.3 $ (1.8)