v3.26.1
Shareholders' Equity - Changes in AOCI by Component (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance $ 5,564 $ 5,657 $ 5,789 $ 5,553
Less: Amounts reclassified from AOCI to:        
Other comprehensive income (loss) 29 87 (83) 158
Ending balance 5,582 5,729 5,582 5,729
Total AOCI        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance (280) (314) (168) (385)
Other comprehensive income (loss) before reclassifications 21 84 (103) 141
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax (11) (4) (26) (20)
Tax (provision) benefit 3 1 6 3
Total amount reclassified from AOCI, net of tax (8) (3) (20) (17)
Other comprehensive income (loss) 29 87 (83) 158
Ending balance (251) (227) (251) (227)
Total AOCI | Location, Statement of Income, Balance [Axis]: us-gaap:RealizedInvestmentGainsLosses        
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax (10) (3) (25) (19)
Total AOCI | Location, Statement of Income, Balance [Axis]: ago:VariableInterestEntityChangeinFairValue        
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax (1) (1) (1) (1)
No Credit Impairment        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance (132) (183) (48) (235)
Other comprehensive income (loss) before reclassifications 2 85 (81) 131
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax (8) 0 (7) (6)
Tax (provision) benefit 2 0 2 0
Total amount reclassified from AOCI, net of tax (6) 0 (5) (6)
Other comprehensive income (loss) 8 85 (76) 137
Ending balance (124) (98) (124) (98)
Credit Impairment        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance (116) (78) (72) (99)
Other comprehensive income (loss) before reclassifications 27 (3) (30) 10
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax (2) (3) (18) (13)
Tax (provision) benefit 1 1 4 3
Total amount reclassified from AOCI, net of tax (1) (2) (14) (10)
Other comprehensive income (loss) 28 (1) (16) 20
Ending balance (88) (79) (88) (79)
ISCR on FG VIEs’ Liabilities with Recourse        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance (17) (19) (17) (18)
Other comprehensive income (loss) before reclassifications 0 0 0 (1)
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax (1) (1) (1) (1)
Tax (provision) benefit 0 0 0 0
Total amount reclassified from AOCI, net of tax (1) (1) (1) (1)
Other comprehensive income (loss) 1 1 1 0
Ending balance (16) (18) (16) (18)
Change in Discount Rates on the LFPB        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance 17   0  
Other comprehensive income (loss) before reclassifications (8)   9  
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax 0   0  
Tax (provision) benefit 0   0  
Total amount reclassified from AOCI, net of tax 0   0  
Other comprehensive income (loss) (8)   9  
Ending balance 9   9  
Cumulative Translation Adjustment        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance (36) (38) (35) (37)
Other comprehensive income (loss) before reclassifications 0 2 (1) 1
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax 0 0 0 0
Tax (provision) benefit 0 0 0 0
Total amount reclassified from AOCI, net of tax 0 0 0 0
Other comprehensive income (loss) 0 2 (1) 1
Ending balance (36) (36) (36) (36)
Cash Flow  Hedge        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance 4 4 4 4
Other comprehensive income (loss) before reclassifications 0 0 0 0
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax 0 0 0 0
Tax (provision) benefit 0 0 0 0
Total amount reclassified from AOCI, net of tax 0 0 0 0
Other comprehensive income (loss) 0 0 0 0
Ending balance $ 4 $ 4 $ 4 $ 4