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Unpaid losses and loss expenses Unpaid losses and loss expenses (Details) (Supplementary PPD Reconciliation) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
supplementary PPD reconciliation [Line Items] |  |  |  |
PPD, Gross | [1] | $ (937) | $ (1,204) | $ (546)
prior period development, net adjustments |  | 108 | 69 | 0
Prior Year Claims and Claims Adjustment Expense |  | (829) | (1,135) | (546)
Long Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (423) | (817) | (263)
Short Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (406) | (318) | (283)
Segments included in loss triangles [Domain] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2] | (122) |  |
PPD, Gross |  | (1,041) |  |
prior period development, net adjustments |  | 53 |  |
Prior Year Claims and Claims Adjustment Expense |  | (988) |  |
Segments included in loss triangles [Domain] | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (658) |  |
Segments included in loss triangles [Domain] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (261) |  |
North America Commercial P&C Insurance |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2],[3] | (73) |  |
PPD, Gross |  | (803) |  |
prior period development, net adjustments |  | 57 |  |
Prior Year Claims and Claims Adjustment Expense |  | (746) | (778) | (264)
North America Commercial P&C Insurance | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (555) |  |
North America Commercial P&C Insurance | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (175) |  |
North America Commercial P&C Insurance | Long Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2] | (76) |  |
PPD, Gross |  | (618) |  |
prior period development, net adjustments |  | 56 |  |
Prior Year Claims and Claims Adjustment Expense |  | (562) | (693) | (162)
North America Commercial P&C Insurance | Long Tail [Member] | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (367) |  |
North America Commercial P&C Insurance | Long Tail [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (175) |  |
North America Commercial P&C Insurance | Short Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2] | 3 |  |
PPD, Gross |  | (185) |  |
prior period development, net adjustments |  | 1 |  |
Prior Year Claims and Claims Adjustment Expense |  | (184) | (85) | (102)
North America Commercial P&C Insurance | Short Tail [Member] | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (188) |  |
North America Commercial P&C Insurance | Short Tail [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 0 |  |
North America Commercial P&C Insurance - Other Casualty [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 0 |  |
North America Commercial P&C Insurance - Other Casualty [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 14 |  |
North America Commercial P&C Insurance - Non-Casualty [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (188) |  |
North America Commercial P&C Insurance - Non-Casualty [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 0 |  |
North America Personal P&C Insurance [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 76 |  |
Prior Year Claims and Claims Adjustment Expense |  | 69 | 27 | 25
North America Personal P&C Insurance [Member] | Alternative Risk Solutions [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (55) |  |
North America Personal P&C Insurance [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (10) |  |
North America Personal P&C Insurance [Member] | Long Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 0 | 0 | 0
North America Personal P&C Insurance [Member] | Short Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2] | (7) |  |
PPD, Gross |  | 69 |  |
prior period development, net adjustments |  | 0 |  |
Prior Year Claims and Claims Adjustment Expense |  | 69 | 27 | 25
North America Personal P&C Insurance [Member] | Short Tail [Member] | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 86 |  |
North America Personal P&C Insurance [Member] | Short Tail [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (10) |  |
Overseas General Insurance |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2],[4] | (43) |  |
PPD, Gross |  | (252) |  |
prior period development, net adjustments |  | 0 |  |
Prior Year Claims and Claims Adjustment Expense |  | (252) | (423) | (343)
Overseas General Insurance | International A&H [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (35) |  |
Overseas General Insurance | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (193) |  |
Overseas General Insurance | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (16) |  |
Overseas General Insurance | Long Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2] | (3) |  |
PPD, Gross |  | (71) |  |
prior period development, net adjustments |  | 0 |  |
Prior Year Claims and Claims Adjustment Expense |  | (71) | (236) | (192)
Overseas General Insurance | Long Tail [Member] | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (55) |  |
Overseas General Insurance | Long Tail [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (13) |  |
Overseas General Insurance | Short Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2] | (40) |  |
PPD, Gross |  | (181) |  |
prior period development, net adjustments |  | 0 |  |
Prior Year Claims and Claims Adjustment Expense |  | (181) | (187) | (151)
Overseas General Insurance | Short Tail [Member] | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (138) |  |
Overseas General Insurance | Short Tail [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (3) |  |
Overseas General Insurance - Casualty [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (68) |  |
Overseas General Insurance - Casualty [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (13) |  |
Overseas General Insurance - Non-Casualty [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (141) |  |
Overseas General Insurance - Non-Casualty [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (3) |  |
Segment Global Reinsurance [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2] | 1 |  |
PPD, Gross |  | (55) |  |
prior period development, net adjustments |  | (4) |  |
Prior Year Claims and Claims Adjustment Expense |  | (59) |  |
Segment Global Reinsurance [Member] | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 4 |  |
Segment Global Reinsurance [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (60) |  |
Segment Global Reinsurance [Member] | Long Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2] | 1 |  |
PPD, Gross |  | (71) |  |
prior period development, net adjustments |  | 3 |  |
Prior Year Claims and Claims Adjustment Expense |  | (68) |  |
Segment Global Reinsurance [Member] | Long Tail [Member] | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (12) |  |
Segment Global Reinsurance [Member] | Long Tail [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (60) |  |
Segment Global Reinsurance [Member] | Short Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Other PPD adjustments | [2] | 0 |  |
PPD, Gross |  | 16 |  |
prior period development, net adjustments |  | (7) |  |
Prior Year Claims and Claims Adjustment Expense |  | 9 |  |
Segment Global Reinsurance [Member] | Short Tail [Member] | 2008 to 2016 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 16 |  |
Segment Global Reinsurance [Member] | Short Tail [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 0 |  |
Global Reinsurance - Casualty [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (72) |  |
Global Reinsurance - Casualty [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | (60) |  |
Global Reinsurance - Non-Casualty [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 16 |  |
Global Reinsurance - Non-Casualty [Member] | prior to 2008 [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
short-duration PPD |  | 0 |  |
North America Agricultural Insurance |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (119) | (72) | (45)
North America Agricultural Insurance | Long Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 0 | 0 | 0
North America Agricultural Insurance | Short Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
PPD, Gross |  | (174) |  |
prior period development, net adjustments |  | 55 |  |
Prior Year Claims and Claims Adjustment Expense |  | (119) | (72) | (45)
Corporate Segment [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 278 | 189 | 200
Corporate Segment [Member] | Long Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
PPD, Gross |  | 278 |  |
prior period development, net adjustments |  | 0 |  |
Prior Year Claims and Claims Adjustment Expense |  | 278 | 189 | 200
Corporate Segment [Member] | Short Tail [Member] |  |  |  |
supplementary PPD reconciliation [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | $ 0 | $ 0 | $ 0
[1] | (2) Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments, and earned premiums totaling $108 million, $69 million, and nil, for 2017, 2016, and 2015, respectively.
[2] | (1) Other includes the impact of foreign exchange.
[3] | (2) Includes favorable development of $55 million related to our Alternative Risk Solutions business; the remaining difference relates to a number of other items, none of which are individually material.
[4] | (3) Includes favorable development of $35 million related to International A&H business, the remaining difference relates to a number of other items, none of which are individually material.

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