Variable Interest Entities (Summary Of Nonconsolidated VIEs Assets And Liabilities) (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	$ 5,099	$ 5,526
Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	12	15
Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	32	34
Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	651	781
Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	32	33
Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	493	447
Global Structured Finance [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	3,376	3,600
Global Structured Finance [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	12	15
Global Structured Finance [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	24	26
Global Structured Finance [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	651	781
Global Structured Finance [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	24	24
Global Structured Finance [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	493	447
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	2,122	2,253
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	12	15
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	17	19
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	119	107
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	16	16
Global Structured Finance [Member] | Residential Mortgage Backed Securities [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	474	436
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	21	26
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member] | Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	0	0
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member] | Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	0	0
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	0	0
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	0	0
Global Structured Finance [Member] | Commercial Mortgage Backed Securities [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	0	0
Global Structured Finance [Member] | Consumer Asset Backed [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	363	384
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	1	1
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	1	1
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	1	1
Global Structured Finance [Member] | Consumer Asset Backed [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	17	11
Global Structured Finance [Member] | Corporate Asset Backed [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	870	937
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	6	6
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	531	673
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	7	7
Global Structured Finance [Member] | Corporate Asset Backed [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	2	0
Global Public Finance [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	1,723	1,926
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	8	8
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	8	9
Global Public Finance [Member] | Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	$ 0	$ 0