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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)

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