v3.25.1
Segment Information - Reconciliation of Revenue to Income (Loss) Before Income Taxes (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Segment Reporting, Asset Reconciling Item [Line Items]      
Net earned premiums $ 210.0 $ 313.7  
Net investment income 59.1 58.8  
Net investment and other gains (losses) 9.7 6.2 $ 6.2
Total revenue 278.8 378.7 378.7
Losses and loss adjustment expenses 148.1 220.0  
Underwriting, acquisition and general expenses 80.6 117.1  
Non-operating expenses 5.6 7.7 7.7
Foreign currency exchange (gains) losses (2.6) (2.0) (2.0)
Net investment and other (gains) losses (9.7) (6.2) (6.2)
Total other expenses (2) 16.1   44.8
Income (loss) before income taxes 38.5 27.2 27.2
Amortization of value of business acquired and other intangible assets 9.9 $ 42.9  
Operating Segments      
Segment Reporting, Asset Reconciling Item [Line Items]      
Net earned premiums 210.0   313.7
Net investment income 59.1   58.8
Total revenue 269.1   372.5
Segment operating income (loss) 47.9   71.5
Casualty Lines | Operating Segments      
Segment Reporting, Asset Reconciling Item [Line Items]      
Net earned premiums 144.1   136.5
Net investment income 30.6   26.9
Total revenue 174.7   163.4
Losses and loss adjustment expenses 90.8   85.9
Underwriting, acquisition and general expenses 44.9   30.2
Other segment items 4.4   4.0
Segment operating income (loss) 34.6   43.3
Specialty Lines | Operating Segments      
Segment Reporting, Asset Reconciling Item [Line Items]      
Net earned premiums 61.0   66.7
Net investment income 4.6   7.1
Total revenue 65.6   73.8
Losses and loss adjustment expenses 46.3   42.3
Underwriting, acquisition and general expenses 17.6   11.9
Other segment items 0.7   1.0
Segment operating income (loss) 1.0   18.6
Run-off Lines | Operating Segments      
Segment Reporting, Asset Reconciling Item [Line Items]      
Net earned premiums 4.9   110.5
Net investment income 23.9   24.8
Total revenue 28.8   135.3
Losses and loss adjustment expenses 11.0   91.8
Underwriting, acquisition and general expenses 2.0   30.2
Other segment items 3.5   3.7
Segment operating income (loss) $ 12.3   $ 9.6