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Claim and Claim Adjustment Expense Reserves (International - 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)
International |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Medical Professional Liability | (4) | (5) | (9)
Other Professional Liability | (13) | 12 | (16)
Liability | 2 | (30) | (17)
Property & Marine | (15) | (34) | (29)
Other | 21 | (1) | 17
Total pretax (favorable) unfavorable development | $ (9) | $ (58) | $ (54)

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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Medical Professional Liability + 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, Other + References No definition available. + Details
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- Definition Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Liability + References No definition available. + Details
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- Definition Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Other Professional Liability + References No definition available. + Details
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- Definition Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Property & Marine + References No definition available. + Details
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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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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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