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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
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense | [1] | $ (981) | $ (937) | $ (1,204)
Prior Period Development, net Adjustments |  | 85 | 108 | 69
Net Prior Period Development |  | (896) | (829) | (1,135)
Alternative Risk Solutions [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (81) |  |
North America Workers' Compensation [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 42 |  |
Long Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (486) | (423) | (817)
Short Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (410) | (406) | (318)
Segments included in loss triangles [Domain] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (886) |  |
Other PPD adjustments | [2] | (205) |  |
Prior Period Development, net Adjustments |  | 55 |  |
Net Prior Period Development |  | (831) |  |
Segments included in loss triangles [Domain] | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (495) |  |
Segments included in loss triangles [Domain] | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (186) |  |
North America Commercial P&C Insurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (653) |  |
Other PPD adjustments | [2],[3] | (153) |  |
Prior Period Development, net Adjustments |  | 43 |  |
Net Prior Period Development |  | (610) | (746) | (778)
North America Commercial P&C Insurance | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (437) |  |
North America Commercial P&C Insurance | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (63) |  |
North America Commercial P&C Insurance | Long Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (425) |  |
Other PPD adjustments | [2] | (149) |  |
Prior Period Development, net Adjustments |  | 30 |  |
Net Prior Period Development |  | (395) | (562) | (693)
North America Commercial P&C Insurance | Long Tail [Member] | North America Workers' Compensation [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (194) | (123) |
North America Commercial P&C Insurance | Long Tail [Member] | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (214) |  |
North America Commercial P&C Insurance | Long Tail [Member] | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (62) |  |
North America Commercial P&C Insurance | Short Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (228) |  |
Other PPD adjustments | [2] | (4) |  |
Prior Period Development, net Adjustments |  | 13 |  |
Net Prior Period Development |  | (215) | (184) | (85)
North America Commercial P&C Insurance | Short Tail [Member] | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (223) |  |
North America Commercial P&C Insurance | Short Tail [Member] | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (1) |  |
North America Personal P&C Insurance [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | 41 | 69 | 27
North America Personal P&C Insurance [Member] | Long Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | 0 | 0 | 0
North America Personal P&C Insurance [Member] | Short Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 40 |  |
Other PPD adjustments | [2] | (7) |  |
Prior Period Development, net Adjustments |  | 1 |  |
Net Prior Period Development |  | 41 | 69 | 27
North America Personal P&C Insurance [Member] | Short Tail [Member] | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 54 |  |
North America Personal P&C Insurance [Member] | Short Tail [Member] | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (7) |  |
Overseas General Insurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (216) |  |
Other PPD adjustments | [2],[4] | (43) |  |
Prior Period Development, net Adjustments |  | 4 |  |
Net Prior Period Development |  | (212) | (252) | (423)
Overseas General Insurance | International A&H [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (31) |  |
Overseas General Insurance | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (107) |  |
Overseas General Insurance | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (66) |  |
Overseas General Insurance | Long Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (67) |  |
Other PPD adjustments | [2] | (3) |  |
Prior Period Development, net Adjustments |  | 0 |  |
Net Prior Period Development |  | (67) | (71) | (236)
Overseas General Insurance | Long Tail [Member] | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (9) |  |
Overseas General Insurance | Long Tail [Member] | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (55) |  |
Overseas General Insurance | Short Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (149) |  |
Other PPD adjustments | [2] | (40) |  |
Prior Period Development, net Adjustments |  | 4 |  |
Net Prior Period Development |  | (145) | (181) | (187)
Overseas General Insurance | Short Tail [Member] | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (98) |  |
Overseas General Insurance | Short Tail [Member] | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (11) |  |
Segment Global Reinsurance [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (57) |  |
Other PPD adjustments | [2] | (2) |  |
Prior Period Development, net Adjustments |  | 7 |  |
Net Prior Period Development |  | (50) | (59) | (78)
Segment Global Reinsurance [Member] | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (5) |  |
Segment Global Reinsurance [Member] | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (50) |  |
Segment Global Reinsurance [Member] | Long Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (74) |  |
Other PPD adjustments | [2] | (1) |  |
Prior Period Development, net Adjustments |  | 5 |  |
Net Prior Period Development |  | (69) | (68) | (77)
Segment Global Reinsurance [Member] | Long Tail [Member] | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (25) |  |
Segment Global Reinsurance [Member] | Long Tail [Member] | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (48) |  |
Segment Global Reinsurance [Member] | Short Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 17 |  |
Other PPD adjustments | [2] | (1) |  |
Prior Period Development, net Adjustments |  | 2 |  |
Net Prior Period Development |  | 19 | 9 | (1)
Segment Global Reinsurance [Member] | Short Tail [Member] | 2009 to 2017 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 20 |  |
Segment Global Reinsurance [Member] | Short Tail [Member] | prior to 2009 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (2) |  |
North America Agricultural Insurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (110) | (119) | (72)
North America Agricultural Insurance | Long Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | 0 | 0 | 0
North America Agricultural Insurance | Short Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (140) |  |
Prior Period Development, net Adjustments |  | 30 |  |
Net Prior Period Development |  | (110) | (119) | (72)
Corporate Segment [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | 45 | 278 | 189
Corporate Segment [Member] | Long Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 45 |  |
Prior Period Development, net Adjustments |  | 0 |  |
Net Prior Period Development |  | 45 | 278 | 189
Corporate Segment [Member] | Short Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | $ 0 | $ 0 | $ 0
[1] | Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments and earned premiums totaling $85 million, $108 million and $69 million for 2018, 2017, and 2016, respectively.
[2] | Other includes the impact of foreign exchange.
[3] | Includes favorable development of $81 million related to our Alternative Risk Solutions business (U.S. and Bermuda) and an adjustment to exclude $42 million in unfavorable development in the workers' compensation line associated with an increase in exposure for which additional premiums were collected; the remaining difference relates to a number of other items, none of which are individually material.
[4] | Includes favorable development of $31 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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