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Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Segment Reporting Information [Line Items]        
Operating revenues $ 1,577.7 $ 1,522.7 $ 4,694.5 $ 4,496.5
Net realized and unrealized investment losses (12.4) (6.1) (41.4) (31.8)
Total revenues 1,565.3 1,516.6 4,653.1 4,464.7
Net investment income 91.8 84.2 271.9 250.5
Operating income (loss) before interest expense and income taxes 150.6 13.7 395.7 (52.5)
Interest on debt (8.5) (8.5) (25.6) (25.6)
Operating income (loss) before income taxes 142.1 5.2 370.1 (78.1)
Other non-operating 0.0   (2.4) 0.8
Income (loss) from continuing operations before income taxes 129.7 (0.9) 326.3 (109.1)
Core Commercial [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 576.6 557.1 1,727.6 1,658.2
Underwriting (loss) income 14.6 5.3 88.3 1.0
Net investment income 42.1 38.4 124.3 114.2
Other income (expense) (0.8) (0.6) (2.0) (0.8)
Operating income (loss) before interest expense and income taxes 55.9 43.1 210.6 114.4
Specialty [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 353.7 340.9 1,049.0 1,011.6
Underwriting (loss) income 52.6 52.7 113.5 119.5
Net investment income 20.9 18.0 61.7 53.6
Other income (expense) (0.5) (0.4) (0.8) (0.1)
Operating income (loss) before interest expense and income taxes 73.0 70.3 174.4 173.0
Personal Lines [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 644.9 620.5 1,908.5 1,815.1
Underwriting (loss) income (6.7) (127.3) (74.4) (422.4)
Net investment income 26.3 24.3 77.9 73.3
Other income (expense) 2.1 2.6 6.7 8.0
Operating income (loss) before interest expense and income taxes 21.7 (100.4) 10.2 (341.1)
Other [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 2.5 4.2 9.4 11.6
Net investment income 2.5 3.5 8.0 9.4
Other income (expense) $ (2.5) (2.8) (7.5) (8.2)
Operating income (loss) before interest expense and income taxes   $ 0.7 $ 0.5 $ 1.2