v3.26.1
Losses, Claims and Settlement Expenses (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Dec. 31, 2024
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]            
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments         $ 6,399.1 $ 5,807.1
Liability for Unpaid Claims and Claims Adjustment Expense            
Gross reserves at beginning of period     $ 14,775.7 $ 13,727.7    
Less: Reinsurance losses recoverable $ 6,757.9 $ 6,236.0 6,757.9 6,236.0    
Net reserves at beginning of period         8,376.5 7,920.6
Provisions for insured events of the current year     1,758.3 1,687.9    
Change in provision for insured events of prior years     (30.6) (88.6)    
Total incurred claims and claim adjustment expenses     1,727.6 1,599.3    
Claims and claim adjustment expenses attributable to insured events of the current year     402.6 393.6    
Claims and claim adjustment expenses attributable to insured events of prior years     1,132.6 1,005.8    
Total payments     1,535.2 1,399.5    
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable 8,568.9 8,120.4 8,568.9 8,120.4    
Gross reserves at end of period 15,326.9 14,356.4 15,326.9 14,356.4    
Premiums Earned, Net 2,024.4 1,926.7 3,937.7 3,709.6    
Financial Indemnity - Transactional Risk Business            
Liability for Unpaid Claims and Claims Adjustment Expense            
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Prior Year Claim and Claim Adjustment Expense     $ 40.0      
Loss ratio point impact from prior year unfavorable reserve development     1.50%      
Specialty Insurance            
Liability for Unpaid Claims and Claims Adjustment Expense            
Net reserves at beginning of period         7,826.6 7,341.5
Provisions for insured events of the current year     $ 1,702.3 1,638.8    
Change in provision for insured events of prior years     (16.6) (77.7)    
Claims and claim adjustment expenses attributable to insured events of the current year     398.1 389.2    
Claims and claim adjustment expenses attributable to insured events of prior years     1,098.4 962.6    
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable 8,015.7 7,550.5 8,015.7 7,550.5    
Title Insurance            
Liability for Unpaid Claims and Claims Adjustment Expense            
Net reserves at beginning of period         545.7 572.7
Provisions for insured events of the current year     53.7 45.4    
Change in provision for insured events of prior years     (12.8) (9.0)    
Claims and claim adjustment expenses attributable to insured events of the current year     3.1 2.6    
Claims and claim adjustment expenses attributable to insured events of prior years     33.0 40.9    
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable 550.4 565.4 550.4 565.4    
Other            
Liability for Unpaid Claims and Claims Adjustment Expense            
Net reserves at beginning of period         $ 4.1 $ 6.4
Provisions for insured events of the current year     2.2 3.7    
Change in provision for insured events of prior years     (1.1) (1.8)    
Claims and claim adjustment expenses attributable to insured events of the current year     1.3 1.7    
Claims and claim adjustment expenses attributable to insured events of prior years     1.1 2.1    
Amount of reserves for unpaid claims and claim adjustment expenses at the end of each period, net of reinsurance losses recoverable $ 2.7 $ 4.3 $ 2.7 $ 4.3