Variable Interest Entities (Summary of Nonconsolidated VIEs Assets and Liabilities) (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	$ 1,607	$ 1,840
Carrying Value of VIE Assets	2,990	3,375
Carrying Value of VIE Liabilities	4,218	4,251
Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	18	75
Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	11	14
Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	27	28
Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	10	12
Loss And Loss Adjustment Expense Reserves [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	222	282
Corporate Asset Backed [Member] | Loss And Loss Adjustment Expense Reserves [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities		0
Global Structured Finance [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	1,391	1,610
Global Structured Finance [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	18	75
Global Structured Finance [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	7	9
Global Structured Finance [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	27	28
Global Structured Finance [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	6	8
Global Structured Finance [Member] | Loss And Loss Adjustment Expense Reserves [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	222	282
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	848	996
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	18	75
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	5	6
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	19	21
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	3	4
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	219	277
Global Structured Finance [Member] | Consumer Asset Backed [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	130	164
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	1	0
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	0	1
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	3	5
Global Structured Finance [Member] | Corporate Asset Backed [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	413	450
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	2	3
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	7	7
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	3	3
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
Global Public Finance [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	216	230
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	4	5
Global Public Finance [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	0	0
Global Public Finance [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	4	4
Global Public Finance [Member] | Loss And Loss Adjustment Expense Reserves [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	$ 0	$ 0