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Claim and Claim Adjustment Expense Reserves (Specialty - 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)
Specialty |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Medical Professional Liability | 5 | (37) | (43)
Other Professional Liability and Management Liability | (131) | (130) | 0
Surety | (84) | (63) | (69)
Warranty | 4 | 4 | (2)
Other | 4 | (61) | (27)
Total pretax (favorable) unfavorable development | $ (202) | $ (287) | $ (141)

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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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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, Surety + References No definition available. + Details
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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Warranty + References No definition available. + Details
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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability + References No definition available. + Details
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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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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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