v3.26.1
Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Segment Reporting Information [Line Items]    
Premiums $ 1,570.6 $ 1,508.5
Net investment income 126.9 106.1
Fees and other income 6.2 6.4
Total operating revenues 1,703.7 1,621.0
Net realized and unrealized investment losses (2.3) (17.8)
Total revenues 1,701.4 1,603.2
Current accident year losses and LAE 883.7 879.7
Prior year favorable development, excluding catastrophes (25.0) (20.0)
Current year catastrophe losses 147.7 107.6
Prior year favorable catastrophe development (48.8) (12.0)
Total losses and LAE 957.6 955.3
Amortization of deferred acquisition costs and other underwriting expenses 486.8 470.3
Underwriting income (loss) 126.2 82.9
Other segment items (9.1) (9.0)
Operating income before interest expense and income taxes 250.2 186.4
Interest on debt (10.8) (8.5)
Operating income before income taxes 239.4 177.9
Income before income taxes 237.1 160.1
Core Commercial [Member]    
Segment Reporting Information [Line Items]    
Premiums 563.8 541.0
Net investment income 58.3 48.0
Fees and other income 1.4 1.3
Total operating revenues 623.5 590.3
Current accident year losses and LAE 331.2 334.2
Prior year favorable development, excluding catastrophes (1.6) (1.3)
Current year catastrophe losses 52.1 54.5
Prior year favorable catastrophe development (21.7) (8.5)
Total losses and LAE 360.0 378.9
Amortization of deferred acquisition costs and other underwriting expenses 186.0 182.1
Underwriting income (loss) 17.8 (20.0)
Other segment items (2.7) (2.5)
Operating income before interest expense and income taxes 74.8 26.8
Specialty [Member]    
Segment Reporting Information [Line Items]    
Premiums 359.9 339.6
Net investment income 28.7 24.3
Fees and other income 1.2 1.3
Total operating revenues 389.8 365.2
Current accident year losses and LAE 176.8 173.3
Prior year favorable development, excluding catastrophes (14.2) (15.9)
Current year catastrophe losses 15.4 17.2
Prior year favorable catastrophe development (5.8) (2.5)
Total losses and LAE 172.2 172.1
Amortization of deferred acquisition costs and other underwriting expenses 131.6 126.3
Underwriting income (loss) 56.1 41.2
Other segment items (2.0) (2.2)
Operating income before interest expense and income taxes 84.0 64.6
Personal Lines [Member]    
Segment Reporting Information [Line Items]    
Premiums 646.9 627.9
Net investment income 35.2 30.5
Fees and other income 3.6 3.7
Total operating revenues 685.7 662.1
Current accident year losses and LAE 375.7 372.2
Prior year favorable development, excluding catastrophes (9.2) (2.8)
Current year catastrophe losses 80.2 35.9
Prior year favorable catastrophe development (21.3) (1.0)
Total losses and LAE 425.4 404.3
Amortization of deferred acquisition costs and other underwriting expenses 169.2 161.9
Underwriting income (loss) 52.3 61.7
Other segment items (1.9) (1.7)
Operating income before interest expense and income taxes 89.2 94.2
Other [Member]    
Segment Reporting Information [Line Items]    
Premiums 0.0 0.0
Net investment income 4.7 3.3
Fees and other income 0.0 0.1
Total operating revenues 4.7 3.4
Current accident year losses and LAE 0.0 0.0
Prior year favorable development, excluding catastrophes (0.0) 0.0
Current year catastrophe losses 0.0 0.0
Prior year favorable catastrophe development 0.0 0.0
Total losses and LAE (0.0) 0.0
Amortization of deferred acquisition costs and other underwriting expenses 0.0 0.0
Underwriting income (loss) 0.0 0.0
Other segment items (2.5) (2.6)
Operating income before interest expense and income taxes $ 2.2 $ 0.8