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Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information [Line Items] |  |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ (308) | $ (288) | $ (210)
Pretax (favorable) unfavorable premium development, excluding Life & Group | (4) | (26) | (8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | (312) | (314) | (218)
Specialty |  |  |
Segment Reporting Information [Line Items] |  |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (202) | (287) | (141)
Pretax (favorable) unfavorable premium development, excluding Life & Group | (14) | (18) | (11)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | (216) | (305) | (152)
Commercial |  |  |
Segment Reporting Information [Line Items] |  |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (87) | 55 | (15)
Pretax (favorable) unfavorable premium development, excluding Life & Group | 28 | (2) | (15)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | (59) | 53 | (30)
International |  |  |
Segment Reporting Information [Line Items] |  |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (9) | (58) | (54)
Pretax (favorable) unfavorable premium development, excluding Life & Group | (18) | (6) | 18
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | (27) | (64) | (36)
Corporate & Other |  |  |
Segment Reporting Information [Line Items] |  |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (10) | 2 | 0
Pretax (favorable) unfavorable premium development, excluding Life & Group | 0 | 0 | 0
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | $ (10) | $ 2 | $ 0

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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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