Variable Interest Entities (Summary Of Nonconsolidated VIEs Assets And Liabilities) (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	$ 4,911	$ 5,526
Carrying Value of VIE Assets	6,551	7,284
Carrying Value of VIE Liabilities	6,308	6,445
Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	21	15
Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	29	34
Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	563	781
Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	29	33
Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	515	447
Global Structured Finance [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	3,021	3,600
Global Structured Finance [Member] \| Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	21	15
Global Structured Finance [Member] \| Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	23	26
Global Structured Finance [Member] \| Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	563	781
Global Structured Finance [Member] \| Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	22	24
Global Structured Finance [Member] \| Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	514	447
Global Structured Finance [Member] \| Residential Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	1,921	2,253
Global Structured Finance [Member] \| Residential Mortgage Backed Securities [Member] \| Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	21	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	104	107
Global Structured Finance [Member] \| Residential Mortgage Backed Securities [Member] \| Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	15	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	495	436
Global Structured Finance [Member] \| Commercial Mortgage Backed Securities [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss		26
Global Structured Finance [Member] \| Commercial Mortgage Backed Securities [Member] \| Investments [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets		0
Global Structured Finance [Member] \| Commercial Mortgage Backed Securities [Member] \| Premiums Receivable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets		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
Global Structured Finance [Member] \| Commercial Mortgage Backed Securities [Member] \| Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities		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
Global Structured Finance [Member] \| Consumer Asset Backed [Member]
Variable Interest Entity [Line Items]
Maximum Exposure to Loss	313	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	787	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	5	6
Global Structured Finance [Member] \| Corporate Asset Backed [Member] \| Insurance Loss Recoverable [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Assets	458	673
Global Structured Finance [Member] \| Corporate Asset Backed [Member] \| Unearned Premium Revenue [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	6	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	1,890	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	6	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	7	9
Global Public Finance [Member] \| Loss And Loss Adjustment Expense Reserves Member [Member]
Variable Interest Entity [Line Items]
Carrying Value of VIE Liabilities	$ 1	$ 0

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized. Liabilities are probable future sacrifices of economic benefits arising from present obligations of an entity to transfer assets or provide services to other entities in the future.

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The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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