v3.26.1
Expected Loss to be Paid (Recovered) - Additional Information (Details)
€ in Millions, £ in Millions, $ in Millions
 1 Months Ended 3 Months Ended
Sep. 18, 2025
board_member
Aug. 31, 2025
board_member
Mar. 31, 2026
USD ($)
Payment
Mar. 31, 2025
USD ($)
Mar. 31, 2026
GBP (£)
Mar. 31, 2026
EUR (€)
Dec. 31, 2025
USD ($)
Schedule of Expected Losses to be Paid [Line Items]              
Period of insured credit performance of guaranteed obligations (in some cases over)     30 years        
Loss and LAE Reserve paid     $ 2 $ (61)      
Expected LAE to be paid     13       $ 8
Credit for estimated future recoveries of claims paid     309       311
Net     278,600       277,100
Insured financial obligations insured contractual payments outstanding, net     441,500       440,800
Plaintiff, terminated board members | board_member   6          
Plaintiff, total board members | board_member   7          
Plaintiff, terminated board members suing for reinstatement | board_member 3            
BIG              
Schedule of Expected Losses to be Paid [Line Items]              
Net     8,640       8,760
European Renewable Energy | BIG              
Schedule of Expected Losses to be Paid [Line Items]              
Net     876     € 758  
Thames Water Utilities Finance PLC | BIG              
Schedule of Expected Losses to be Paid [Line Items]              
Net     2,400   £ 1,800    
U.K. Student Accommodations | BIG              
Schedule of Expected Losses to be Paid [Line Items]              
Net     386   292    
U.K. National Health Service Trusts | BIG              
Schedule of Expected Losses to be Paid [Line Items]              
Net     552   £ 417    
U.S. Transportation | BIG              
Schedule of Expected Losses to be Paid [Line Items]              
Net     $ 1,100        
United States | RMBS | First Lien              
Schedule of Expected Losses to be Paid [Line Items]              
Number of delinquent payments | Payment     2        
Deferred loan balances to be recovered, least stressful environment, percent     80.00%   80.00% 80.00%  
United States | RMBS | First Lien | More Stressful Environment              
Schedule of Expected Losses to be Paid [Line Items]              
Projected loss assumptions, period to reach final loss severity rate     9 years        
Percent of deferred loan balances to be recovered     20.00%        
Period from plateau to intermediate conditional default rate (in months)     16 months        
Projected loss assumptions, increase (decrease) in expected loss to be paid, net     $ 32        
United States | RMBS | First Lien | Least Stressful Environment              
Schedule of Expected Losses to be Paid [Line Items]              
Projected loss assumptions, CDR, plateau rate, projection period     30 months        
Period from plateau to intermediate conditional default rate (in months)     8 months        
Decrease in the plateau period used to calculate potential change in loss estimate (in months)     6 months        
Projected loss assumptions, increase (decrease) in expected loss to be paid, net     $ (31)        
United States | RMBS | Second Lien              
Schedule of Expected Losses to be Paid [Line Items]              
Percent of deferred loan balances to be recovered     2.00%        
Projected loss assumptions, period of consistent conditional default rate     5 years        
United States | RMBS | Second Lien | More Stressful Environment              
Schedule of Expected Losses to be Paid [Line Items]              
Percent of deferred loan balances to be recovered     40.00%        
United States | RMBS | Second Lien | Least Stressful Environment              
Schedule of Expected Losses to be Paid [Line Items]              
Percent of deferred loan balances to be recovered     80.00%        
United States | RMBS | Second Lien | More Stressful Environment | Home Equity Line of Credit              
Schedule of Expected Losses to be Paid [Line Items]              
Change in estimate for increased conditional default rate plateau period     $ 46        
United States | RMBS | Second Lien | Least Stressful Environment | Home Equity Line of Credit              
Schedule of Expected Losses to be Paid [Line Items]              
Change in estimate for decreased conditional default rate ramp down period     47        
Puerto Rico              
Schedule of Expected Losses to be Paid [Line Items]              
Net     553       553
Insured financial obligations insured contractual payments outstanding, net     632       643
Puerto Rico | Other Puerto Rico Exposure              
Schedule of Expected Losses to be Paid [Line Items]              
Net     76       76
Puerto Rico | Puerto Rico Defaulted Exposures Subject to a Plan Support Agreement | Puerto Rico Electric Power Authority              
Schedule of Expected Losses to be Paid [Line Items]              
Net     464       464
Insured financial obligations insured contractual payments outstanding, net     $ 528       $ 537