| **Property Casualty Loss and Loss Expenses - Consolidated Property Casualty Loss and Loss Expense Reserves (Detail) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Summary of Consolidated Property Casualty Loss and Loss Expense Reserves [Roll Forward]**                                                                          |           |           |   |   |
| Gross loss and loss expense reserves, beginning of period                                                                                                            | $ 4,219        | $ 4,337        | $ 4,169 | $ 4,280 |
| Less reinsurance receivable                                                                                                                                          | 333            | 332            | 356 | 375 |
| Net loss and loss expense reserves, beginning of period                                                                                                              | 3,886          | 4,005          | 3,813 | 3,905 |
| Current accident year                                                                                                                                                | 631            | 611            | 1,840 | 1,991 |
| Prior accident years                                                                                                                                                 | (38)           | (86)           | (140) | (287) |
| Total incurred                                                                                                                                                       | 593            | 525            | 1,700 | 1,704 |
| Current accident year                                                                                                                                                | 329            | 364            | 699 | 778 |
| Prior accident years                                                                                                                                                 | 233            | 229            | 897 | 894 |
| Total paid                                                                                                                                                           | 562            | 593            | 1,596 | 1,672 |
| Net loss and loss expense reserves, end of period                                                                                                                    | 3,917          | 3,937          | 3,917 | 3,937 |
| Plus reinsurance receivable                                                                                                                                          | 321            | 343            | 321 | 343 |
| Gross loss and loss expense reserves, end of period                                                                                                                  | $ 4,238        | $ 4,280        | $ 4,238 | $ 4,280 |

| X                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recovery of Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years
+ References
No definition available.
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| X                                                                                                                                                                                                        |
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total amount of payments made during the reporting period to settle insured claims and pay costs incurred in the claims settlement process.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of payments made in the reporting period to settle claims incurred in the current period and related claims settlement costs.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of payments made in the reporting period to settle claims incurred in prior periods and related claims settlement costs.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The estimated cost of settling claims under the terms of the underlying insurance policies and assumed and ceded insurance contracts as of the balance sheet date, including an estimate for claims which have been incurred but not reported and the actual and estimated costs of settling claims.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total provision in the period for claims incurred and costs incurred in the claim settlement process.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of provision in the period for claims incurred in the reporting period and related claims settlement costs.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, amounts expected of incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.
+ References
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