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Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Segment Reporting Information [Line Items]      
Premiums $ 6,161.1 $ 5,912.6 $ 5,663.1
Net investment income 454.4 372.6 332.1
Fees and other income 24.9 28.0 30.8
Total operating revenues 6,640.4 6,313.2 6,026.0
Net realized and unrealized investment losses (46.0) (75.8) (32.5)
Total revenues 6,594.4 6,237.4 5,993.5
Current accident year losses and LAE 3,523.4 3,448.9 3,460.4
Prior year unfavorable (favorable) development, excluding catastrophes (70.4) (67.4) (15.9)
Catastrophe Losses 302.3 417.9 690.1
Prior year unfavorable (favorable) catastrophe development   (42.0)  
Prior year favorable catastrophe development (26.0)    
Total losses and LAE 3,729.3 3,757.4 4,134.6
Amortization of deferred acquisition costs and other underwriting expenses 1,939.3 1,870.7 1,752.9
Underwriting Income (Loss), Total 492.5 284.5 (224.4)
Other segment items (38.8) (35.0) (32.9)
Operating income (loss) before interest expense and income taxes 933.0 650.1 105.6
Interest on debt (43.2) (34.1) (34.1)
Operating income before income taxes 889.8 616.0 71.5
Other non-operating items   (2.4) 2.1
Income from continuing operations before income taxes 843.8 537.8 41.1
Core Commercial [Member]      
Segment Reporting Information [Line Items]      
Premiums 2,211.0 2,148.8 2,060.3
Net investment income 202.5 170.4 151.8
Fees and other income 5.2 5.0 4.7
Total operating revenues 2,418.7 2,324.2 2,216.8
Current accident year losses and LAE 1,305.6 1,242.6 1,178.9
Prior year unfavorable (favorable) development, excluding catastrophes (7.1) (17.7) 4.7
Catastrophe Losses 124.4 109.5 178.0
Prior year unfavorable (favorable) catastrophe development   (32.3) (6.7)
Prior year favorable catastrophe development (11.0)    
Total losses and LAE 1,411.9 1,302.1 1,354.9
Amortization of deferred acquisition costs and other underwriting expenses 746.9 731.9 688.8
Underwriting Income (Loss), Total 52.2 114.8 16.6
Other segment items (9.0) (8.6) (5.9)
Operating income (loss) before interest expense and income taxes 250.9 281.6 167.2
Specialty [Member]      
Segment Reporting Information [Line Items]      
Premiums 1,398.3 1,322.0 1,274.2
Net investment income 102.6 84.5 71.1
Fees and other income 4.8 5.9 6.4
Total operating revenues 1,505.7 1,412.4 1,351.7
Current accident year losses and LAE 699.6 655.4 651.2
Prior year unfavorable (favorable) development, excluding catastrophes (56.8) (46.2) (48.8)
Catastrophe Losses 38.1 52.2 51.4
Prior year unfavorable (favorable) catastrophe development   (14.7) (8.3)
Prior year favorable catastrophe development (4.0)    
Total losses and LAE 676.9 646.7 645.5
Amortization of deferred acquisition costs and other underwriting expenses 524.1 500.1 455.8
Underwriting Income (Loss), Total 197.3 175.2 172.9
Other segment items 8.6 (7.9) (6.9)
Operating income (loss) before interest expense and income taxes 296.1 257.7 243.5
Personal Lines [Member]      
Segment Reporting Information [Line Items]      
Premiums 2,551.8 2,441.8 2,328.6
Net investment income 128.5 106.7 96.8
Fees and other income 14.7 15.7 16.7
Total operating revenues 2,695.0 2,564.2 2,442.1
Current accident year losses and LAE 1,518.2 1,550.9 1,630.3
Prior year unfavorable (favorable) development, excluding catastrophes (7.2) (4.9) 25.9
Catastrophe Losses 139.8 256.2 460.7
Prior year unfavorable (favorable) catastrophe development   5.0 15.0
Prior year favorable catastrophe development (11.0)    
Total losses and LAE 1,639.8 1,807.2 2,131.9
Amortization of deferred acquisition costs and other underwriting expenses 668.3 638.7 608.3
Underwriting Income (Loss), Total 243.7 (4.1) (411.6)
Other segment items (7.1) (7.0) (6.2)
Operating income (loss) before interest expense and income taxes 379.8 111.3 (304.3)
Other [Member]      
Segment Reporting Information [Line Items]      
Net investment income 20.8 11.0 12.4
Fees and other income 0.2 1.4 3.0
Total operating revenues 21.0 12.4 15.4
Prior year unfavorable (favorable) development, excluding catastrophes 0.7 1.4 2.3
Total losses and LAE 0.7 1.4 2.3
Underwriting Income (Loss), Total (0.7) (1.4) (2.3)
Other segment items (14.1) (11.5) (13.9)
Operating income (loss) before interest expense and income taxes $ 6.2 $ (0.5) $ (0.8)