| **Property Casualty Loss and Loss Expenses  - Additional Information (Detail) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |         |         |         |         |
|---------------------------------------------------------------------------------------------------------|----------------|----------------|---------|---------|---------|---------|
||
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Reserve for loss and loss expenses                                                                      | $ 4,937        | $ 4,653        | $ 4,937 | $ 4,653 | $ 4,918 | $ 4,660 | $ 4,647 | $ 4,438 |
| Prior accident years                                                                                    | (40)           | (48)           | (151)   | (140)   |    |    |   |   |
| Life and Health Loss Reserves                                                                           |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Reserve for loss and loss expenses                                                                      | 54             | 47             | 54      | 47      |    |    |   |   |
| Commercial Lines                                                                                        |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (31)           | (43)           | (118)   | (120)   |    |    |   |   |
| Commercial Lines | Workers' Compensation                                                                |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (16)           | (25)           | (52)    | (80)    |    |    |   |   |
| Commercial Lines | Commercial Casualty Segment [Member]                                                 |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (7)            | (16)           | (30)    | (36)    |    |    |   |   |
| Commercial Lines | Other Commercial Lines [Member]                                                      |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (11)           |           | (37)    | (17)    |    |    |   |   |
| Commercial Lines | Commercial Auto                                                                      |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | 4              | 4              | 26      | 27      |    |    |   |   |
| Commercial Lines | Commercial Property Segment [Member]                                                 |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    |           |           | (25)    |    |    |    |   |   |
| Personal Lines                                                                                          |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | 4              | 2              | (4)     | 1       |    |    |   |   |
| Personal Lines | Personal Auto [Member]                                                                 |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | 9              |           | 15      |    |    |    |   |   |
| Excess and Surplus Lines Insurance                                                                      |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | (12)           | $ (7)          | (27)    | (21)    |    |    |   |   |
| Reinsurance assumed and other non segment [Member]                                                      |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    | $ (1)          |           | (2)     |    |    |    |   |   |
| Consolidated Property and Casualty Insurance Entity | Catastrophe Development                           |           |           |    |    |    |    |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                 |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                    |           |           | $ (5)   | $ (12)  |    |    |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
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| X                                                                                                                                                         |
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