v3.26.3
Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Schedule of Insured Financial Obligations [Line Items]    
Net $ 281,100 $ 277,100
% of total net par outstanding 100.00% 100.00%
AAA    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 2,000 $ 2,800
% of total net par outstanding 0.70% 1.00%
AA    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 28,400 $ 25,000
% of total net par outstanding 10.10% 9.00%
A    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 142,700 $ 140,000
% of total net par outstanding 50.80% 50.50%
BBB    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 100,000 $ 100,500
% of total net par outstanding 35.50% 36.30%
BIG    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 8,020 $ 8,760
% of total net par outstanding 2.90% 3.20%
Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 268,000 $ 265,600
Public finance | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 219,700 $ 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 $ 48,300 $ 50,600
% of total net par outstanding 100.00% 100.00%
Public finance | AAA | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 100 $ 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 $ 1,200 $ 1,800
% of total net par outstanding 2.50% 3.50%
Public finance | AA | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 19,900 $ 18,200
% of total net par outstanding 9.10% 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 $ 127,400 $ 122,800
% of total net par outstanding 58.00% 57.10%
Public finance | A | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 11,200 $ 13,500
% of total net par outstanding 23.20% 26.70%
Public finance | BBB | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 69,100 $ 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 63.10% 58.10%
Public finance | BIG | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 3,200 $ 3,500
% of total net par outstanding 1.40% 1.60%
Public finance | BIG | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 4,000 $ 4,400
% of total net par outstanding 8.30% 8.80%
Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 13,100 $ 11,500
Structured finance | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 8,700 $ 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 $ 400 $ 500
% of total net par outstanding 4.40% 5.80%
Structured finance | AAA | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 300 $ 500
% of total net par outstanding 7.10% 12.60%
Structured finance | AA | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 6,000 $ 5,200
% of total net par outstanding 69.20% 66.40%
Structured finance | AA | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 1,100 $ 100
% of total net par outstanding 24.70% 3.50%
Structured finance | A | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 1,200 $ 700
% of total net par outstanding 13.70% 8.90%
Structured finance | A | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 2,900 $ 3,000
% of total net par outstanding 66.90% 83.70%
Structured finance | BBB | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 300 $ 600
% of total net par outstanding 3.50% 8.20%
Structured finance | BBB | Non-U.S. public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 100 $ 0
% of total net par outstanding 1.30% 0.20%
Structured finance | BIG | United States    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 800 $ 900
% of total net par outstanding 9.20% 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%