v3.26.3
Shareholders' Equity - Changes in AOCI by Component (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance $ 5,582 $ 5,729 $ 5,789 $ 5,553
Less: Amounts reclassified from AOCI to:        
Other comprehensive income (loss) (215) 48 (298) 206
Ending balance 5,428 5,737 5,428 5,737
Total AOCI        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance (251) (227) (168) (385)
Other comprehensive income (loss) before reclassifications (224) 39 (327) 180
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax (12) (11) (38) (31)
Tax (provision) benefit 3 2 9 5
Total amount reclassified from AOCI, net of tax (9) (9) (29) (26)
Other comprehensive income (loss) (215) 48 (298) 206
Ending balance (466) (179) (466) (179)
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 (11) (10) (36) (29)
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) (2) (2)
No Credit Impairment        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance (124) (98) (48) (235)
Other comprehensive income (loss) before reclassifications (225) 38 (306) 169
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax (3) 1 (10) (5)
Tax (provision) benefit 1 0 3 0
Total amount reclassified from AOCI, net of tax (2) 1 (7) (5)
Other comprehensive income (loss) (223) 37 (299) 174
Ending balance (347) (61) (347) (61)
Credit Impairment        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance (88) (79) (72) (99)
Other comprehensive income (loss) before reclassifications (17) 0 (47) 10
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax (8) (11) (26) (24)
Tax (provision) benefit 2 2 6 5
Total amount reclassified from AOCI, net of tax (6) (9) (20) (19)
Other comprehensive income (loss) (11) 9 (27) 29
Ending balance (99) (70) (99) (70)
ISCR on FG VIEs’ Liabilities with Recourse        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance (16) (18) (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) (2) (2)
Tax (provision) benefit 0 0 0 0
Total amount reclassified from AOCI, net of tax (1) (1) (2) (2)
Other comprehensive income (loss) 1 1 2 1
Ending balance (15) (17) (15) (17)
Change in Discount Rates on the LFPB        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance 9   0  
Other comprehensive income (loss) before reclassifications 19   28  
Less: Amounts reclassified from AOCI to:        
Reclassification from accumulated other comprehensive income, current period, before tax 0   0  
Tax (provision) benefit   0  
Total amount reclassified from AOCI, net of tax 0   0  
Other comprehensive income (loss) 19   28  
Ending balance 28   28  
Cumulative Translation Adjustment        
AOCI Including Portion Attributable to Noncontrolling Interest [Abstract]        
Beginning balance (36) (36) (35) (37)
Other comprehensive income (loss) before reclassifications (1) 1 (2) 2
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) (1) 1 (2) 2
Ending balance (37) (35) (37) (35)
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