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Summary of Business Segment Information (Parenthetical) (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Commercial Lines |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Federal excise tax relating to cessions from Insurance Operations to Reinsurance Operations | $ 258 | $ 277 | $ 787 | $ 838
Personal Lines |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Federal excise tax relating to cessions from Insurance Operations to Reinsurance Operations | $ 310 |  | $ 957 |

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- Definition Federal Excise Tax Expense Cessions From Insurance Operations To Reinsurance Operations + References No definition available. + Details
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Balance Type: | debit
Period Type: | duration

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- 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. + Details
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |