v3.26.1
Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Schedule of Insured Financial Obligations [Line Items]    
Net $ 278,600 $ 277,100
% of total net par outstanding 100.00% 100.00%
AAA    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 2,700 $ 2,800
% of total net par outstanding 1.00% 1.00%
AA    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 26,000 $ 25,000
% of total net par outstanding 9.30% 9.00%
A    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 142,100 $ 140,000
% of total net par outstanding 51.00% 50.50%
BBB    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 99,200 $ 100,500
% of total net par outstanding 35.60% 36.30%
BIG    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 8,640 $ 8,760
% of total net par outstanding 3.10% 3.20%
Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 265,900 $ 265,600
Public finance | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 216,400 $ 215,000
% of total net par outstanding 100.00% 100.00%
Public finance | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 49,500 $ 50,600
% of total net par outstanding 100.00% 100.00%
Public finance | AAA | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 0 $ 0
% of total net par outstanding 0.00% 0.00%
Public finance | AAA | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 1,800 $ 1,800
% of total net par outstanding 3.60% 3.50%
Public finance | AA | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 18,600 $ 18,200
% of total net par outstanding 8.60% 8.50%
Public finance | AA | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 1,400 $ 1,500
% of total net par outstanding 2.90% 2.90%
Public finance | A | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 126,300 $ 122,800
% of total net par outstanding 58.30% 57.10%
Public finance | A | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 11,400 $ 13,500
% of total net par outstanding 23.00% 26.70%
Public finance | BBB | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 68,100 $ 70,500
% of total net par outstanding 31.50% 32.80%
Public finance | BBB | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 30,500 $ 29,400
% of total net par outstanding 61.70% 58.10%
Public finance | BIG | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 3,400 $ 3,500
% of total net par outstanding 1.60% 1.60%
Public finance | BIG | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 4,400 $ 4,400
% of total net par outstanding 8.80% 8.80%
Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 12,700 $ 11,500
Structured finance | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 8,300 $ 7,900
% of total net par outstanding 100.00% 100.00%
Structured finance | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 4,400 $ 3,600
% of total net par outstanding 100.00% 100.00%
Structured finance | AAA | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 500 $ 500
% of total net par outstanding 5.30% 5.80%
Structured finance | AAA | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 400 $ 500
% of total net par outstanding 10.10% 12.60%
Structured finance | AA | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 5,400 $ 5,200
% of total net par outstanding 64.70% 66.40%
Structured finance | AA | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 600 $ 100
% of total net par outstanding 13.10% 3.50%
Structured finance | A | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 1,000 $ 700
% of total net par outstanding 13.10% 8.90%
Structured finance | A | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 3,400 $ 3,000
% of total net par outstanding 76.60% 83.70%
Structured finance | BBB | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 600 $ 600
% of total net par outstanding 7.00% 8.20%
Structured finance | BBB | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 0 $ 0
% of total net par outstanding 0.20% 0.20%
Structured finance | BIG | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 800 $ 900
% of total net par outstanding 9.90% 10.70%
Structured finance | BIG | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 0 $ 0
% of total net par outstanding 0.00% 0.00%