| **Reinsurance - Incurred Consolidated Property Casualty Insurance Loss and Loss Expenses on Assumed and Ceded Business (Detail) (USD $)In Millions, unless otherwise specified** | 3 Months Ended                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]**                                                                                                |                                                                                                                                                                      |   |
| Incurred loss and loss expenses                                                                                                                                                  | $ 749us-gaap_PolicyholderBenefitsAndClaimsIncurredNet                                                                                                                     | $ 732us-gaap_PolicyholderBenefitsAndClaimsIncurredNet |
| Consolidated Property and Casualty Insurance Entity                                                                                                                              |                                                                                                                                                                      |   |
| **Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]**                                                                                                |                                                                                                                                                                      |   |
| Direct incurred loss and loss expenses                                                                                                                                           | 691us-gaap_PolicyholderBenefitsAndClaimsIncurredGross/ us-gaap_StatementBusinessSegmentsAxis= us-gaap_ConsolidatedPropertyAndCasualtyInsuranceEntityMember                | 677us-gaap_PolicyholderBenefitsAndClaimsIncurredGross/ us-gaap_StatementBusinessSegmentsAxis= us-gaap_ConsolidatedPropertyAndCasualtyInsuranceEntityMember |
| Assumed incurred loss and loss expenses                                                                                                                                          | (1)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed/ us-gaap_StatementBusinessSegmentsAxis= us-gaap_ConsolidatedPropertyAndCasualtyInsuranceEntityMember | 2us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed/ us-gaap_StatementBusinessSegmentsAxis= us-gaap_ConsolidatedPropertyAndCasualtyInsuranceEntityMember |
| Ceded incurred loss and loss expenses                                                                                                                                            | (1)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded/ us-gaap_StatementBusinessSegmentsAxis= us-gaap_ConsolidatedPropertyAndCasualtyInsuranceEntityMember   | (3)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded/ us-gaap_StatementBusinessSegmentsAxis= us-gaap_ConsolidatedPropertyAndCasualtyInsuranceEntityMember |
| Incurred loss and loss expenses                                                                                                                                                  | $ 689us-gaap_PolicyholderBenefitsAndClaimsIncurredNet/ us-gaap_StatementBusinessSegmentsAxis= us-gaap_ConsolidatedPropertyAndCasualtyInsuranceEntityMember                | $ 676us-gaap_PolicyholderBenefitsAndClaimsIncurredNet/ us-gaap_StatementBusinessSegmentsAxis= us-gaap_ConsolidatedPropertyAndCasualtyInsuranceEntityMember |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before effect of policies assumed or ceded, of expense related to provision for policy benefits and costs incurred.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 5 -Article 7
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 5 -Article 7Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of policy benefits and costs incurred for policies assumed.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of reduction of provision for policy benefits and costs incurred for policies ceded.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387
+ Details**Name:**us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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