v3.26.1
Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Schedule of Insured Financial Obligations [Line Items]    
Net $ 278,600 $ 277,100
Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 265,900 265,600
Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 12,700 11,500
BIG    
Schedule of Insured Financial Obligations [Line Items]    
Net 8,640 8,760
BIG | BIG 1    
Schedule of Insured Financial Obligations [Line Items]    
Net 3,630 3,740
BIG | BIG 1 | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 3,450 3,570
BIG | BIG 1 | Other structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 0 0
BIG | BIG 1 | Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 180 170
BIG | BIG 2    
Schedule of Insured Financial Obligations [Line Items]    
Net 3,830 3,810
BIG | BIG 2 | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 3,790 3,770
BIG | BIG 2 | Other structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 20 10
BIG | BIG 2 | Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 40 40
BIG | BIG 3    
Schedule of Insured Financial Obligations [Line Items]    
Net 1,180 1,210
BIG | BIG 3 | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 580 580
BIG | BIG 3 | Other structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 50 60
BIG | BIG 3 | Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 600 630
BIG | Total BIG    
Schedule of Insured Financial Obligations [Line Items]    
Net 8,640 8,760
BIG | Total BIG | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 7,820 7,920
BIG | Total BIG | Other structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 70 70
BIG | Total BIG | Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 820 840
United States | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 216,400 215,000
United States | Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 8,300 7,900
United States | BIG | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 3,400 3,500
United States | BIG | Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 800 900
United States | BIG | BIG 1 | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 2,410 2,480
United States | BIG | BIG 1 | RMBS    
Schedule of Insured Financial Obligations [Line Items]    
Net 180 170
United States | BIG | BIG 2 | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 470 420
United States | BIG | BIG 2 | RMBS    
Schedule of Insured Financial Obligations [Line Items]    
Net 20 30
United States | BIG | BIG 3 | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 580 580
United States | BIG | BIG 3 | RMBS    
Schedule of Insured Financial Obligations [Line Items]    
Net 550 570
United States | BIG | Total BIG | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 3,460 3,480
United States | BIG | Total BIG | RMBS    
Schedule of Insured Financial Obligations [Line Items]    
Net 750 770
Non-U.S. public finance | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 49,500 50,600
Non-U.S. public finance | Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 4,400 3,600
Non-U.S. public finance | BIG | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 4,400 4,400
Non-U.S. public finance | BIG | Structured finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 0 0
Non-U.S. public finance | BIG | BIG 1 | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 1,040 1,090
Non-U.S. public finance | BIG | BIG 2 | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 3,320 3,350
Non-U.S. public finance | BIG | BIG 3 | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net 0 0
Non-U.S. public finance | BIG | Total BIG | Public finance    
Schedule of Insured Financial Obligations [Line Items]    
Net $ 4,360 $ 4,440