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Claim and Claim Adjustment Expense Reserves (Commercial - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Total pretax (favorable) unfavorable development | $ (308) | $ (288) | $ (210)
Commercial |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Commercial Auto | (38) | (46) | (22)
General Liability | 0 | (28) | (33)
Workers' Compensation | (65) | 150 | 80
Property and Other | 16 | (21) | (40)
Total pretax (favorable) unfavorable development | $ (87) | $ 55 | $ (15)

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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Net prior year claim and allocated claim adjustment expense reserve development, General Liability + References No definition available. + Details
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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Property and Other + References No definition available. + Details
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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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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Period Type: |