v3.26.1
RESERVE FOR LOSSES AND LAE - Narrative (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Oct. 01, 2025
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2025
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses   $ 2,250 $ 2,893  
Prior year losses   (33)    
Funds held under reinsurance treaties   272   $ 267
Funds held by reinsureds   1,395   1,326
Incurred losses and LAE   2,217 2,893  
Reinsurance recoverables, adverse development reinsurance agreement       1,250
Reinsurance Contract [Axis]: Adverse Development Cover Reinsurance Agreements        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Funds held under reinsurance treaties   5,400    
Deferred gain on sale current   2   3
Reinsurance recoverables, adverse development reinsurance agreement   1,250    
Policyholder benefits and claims incurred, ceded   1,250   1,250
State National Reinsurer | Reinsurance Contract [Axis]: Adverse Development Cover Reinsurance Agreements        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Payments for reinsurance $ 1,300      
Reinsurance, amount retained, per policy $ 1,300      
Reinsurance, excess retention (in percent) 85.70%      
Reinsurance, excess retention, amount reinsured, per policy $ 600      
Funds held by reinsureds $ 250      
Reinsurance, excess retention unexpired limit, amount reinsured, per policy   598   597
MS Transverse Reinsurance | Reinsurance Contract [Axis]: Adverse Development Cover Reinsurance Agreements        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Payments for reinsurance   $ 122    
Reinsurance, excess retention (in percent)   80.00%    
Reinsurance, excess retention, amount reinsured, per policy   $ 400    
Incurred losses and LAE   122    
Reinsurance, excess retention unexpired limit, amount reinsured, per policy   400   $ 400
Higher Premiums Earned        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses   (239)    
Catastrophe        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses   (404)    
Washington D.C. Aviation Incident        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses     83  
Attritional Losses        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Prior year losses   (70)    
Russia/Ukraine Losses        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Prior year losses   37    
Catastrophe Losses        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses   130 534  
Middle East Conflict        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses   58    
Winter Storm Fern        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses   27    
U.S. Winter Weather Events        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses   20    
Kristin Storms        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses   15    
Nils Storms        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses   $ 10    
Southern California wildfires        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses     512  
Myanmar Earthquake        
Reserve For Losses LAE And Future Policy Benefit Reserve [Line Items]        
Current year losses     $ 22