v3.26.1
Variable Interest Entities (Summary of Nonconsolidated VIEs Assets and Liabilities) (Detail) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Mar. 31, 2025
Variable Interest Entity [Line Items]      
Maximum Exposure to Loss $ 1,152 $ 1,188  
Carrying Value of VIE Assets 1,979 2,013 $ 2,132
Carrying Value of VIE Liabilities 4,257 4,243  
Unearned Premium Revenue [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 6 6  
Loss And Loss Adjustment Expense Reserves [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 229 235  
Investments [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 28 29  
Premiums Receivable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 8 8  
Insurance Loss Recoverable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 22 22  
Global Structured Finance [Member]      
Variable Interest Entity [Line Items]      
Maximum Exposure to Loss 971 999  
Global Structured Finance [Member] | Unearned Premium Revenue [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 4 4  
Global Structured Finance [Member] | Loss And Loss Adjustment Expense Reserves [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 229 235  
Global Structured Finance [Member] | Investments [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 28 29  
Global Structured Finance [Member] | Premiums Receivable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets   5  
Global Structured Finance [Member] | Insurance Loss Recoverable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 22 22  
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member]      
Variable Interest Entity [Line Items]      
Maximum Exposure to Loss 714 729  
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Unearned Premium Revenue [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 3 3  
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Loss And Loss Adjustment Expense Reserves [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 229 234  
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Investments [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 28 29  
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Premiums Receivable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 4 4  
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Insurance Loss Recoverable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 22 22  
Global Structured Finance [Member] | Consumer Asset Backed [Member]      
Variable Interest Entity [Line Items]      
Maximum Exposure to Loss 52 58  
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Unearned Premium Revenue [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 0 0  
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Loss And Loss Adjustment Expense Reserves [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 0 1  
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Investments [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 0 0  
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Premiums Receivable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 0 0  
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Insurance Loss Recoverable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 0 0  
Global Structured Finance [Member] | Corporate Asset Backed [Member]      
Variable Interest Entity [Line Items]      
Maximum Exposure to Loss 205 212  
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Unearned Premium Revenue [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 1 1  
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Loss And Loss Adjustment Expense Reserves [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 0 0  
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Investments [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 0 0  
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Premiums Receivable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 1 1  
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Insurance Loss Recoverable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 0 0  
Global Public Finance [Member]      
Variable Interest Entity [Line Items]      
Maximum Exposure to Loss 181 189  
Global Public Finance [Member] | Unearned Premium Revenue [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 2 2  
Global Public Finance [Member] | Loss And Loss Adjustment Expense Reserves [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Liabilities 0 0  
Global Public Finance [Member] | Investments [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 0 0  
Global Public Finance [Member] | Premiums Receivable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets 3 3  
Global Public Finance [Member] | Insurance Loss Recoverable [Member]      
Variable Interest Entity [Line Items]      
Carrying Value of VIE Assets $ 0 $ 0