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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, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense | [1] | $ (845) | $ (981) | $ (937)
Prior Period Development, net Adjustments |  | 53 | 85 | 108
Net Prior Period Development |  | (792) | (896) | (829)
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 |  | (82) |  |
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 |  | 22 |  |
Long Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (642) | (486) | (423)
Short Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (150) | (410) | (406)
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 |  | (895) |  |
Other PPD adjustments | [2] | (156) |  |
Prior Period Development, net Adjustments |  | 30 |  |
Net Prior Period Development |  | (865) |  |
Segments included in loss triangles [Domain] | prior to 2010 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (215) |  |
Segments included in loss triangles [Domain] | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (524) |  |
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 |  | (683) |  |
Other PPD adjustments | [2],[3] | (118) |  |
Prior Period Development, net Adjustments |  | 34 |  |
Net Prior Period Development |  | (649) | (610) | (746)
North America Commercial P&C Insurance | prior to 2010 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (143) |  |
North America Commercial P&C Insurance | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (422) |  |
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 |  | (707) |  |
Other PPD adjustments | [2] | (110) |  |
Prior Period Development, net Adjustments |  | 39 |  |
Net Prior Period Development |  | (668) | (395) | (562)
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 |  | (303) | (194) |
North America Commercial P&C Insurance | Long Tail [Member] | prior to 2010 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (137) |  |
North America Commercial P&C Insurance | Long Tail [Member] | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (460) |  |
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 |  | 24 |  |
Other PPD adjustments | [2] | (8) |  |
Prior Period Development, net Adjustments |  | (5) |  |
Net Prior Period Development |  | 19 | (215) | (184)
North America Commercial P&C Insurance | Short Tail [Member] | prior to 2010 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (6) |  |
North America Commercial P&C Insurance | Short Tail [Member] | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 38 |  |
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 |  | (95) | 41 | 69
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 |  | (91) |  |
Other PPD adjustments | [2] | (5) |  |
Prior Period Development, net Adjustments |  | (4) |  |
Net Prior Period Development |  | (95) | 41 | 69
North America Personal P&C Insurance [Member] | Short Tail [Member] | prior to 2010 [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] | Short Tail [Member] | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (85) |  |
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 |  | (93) |  |
Other PPD adjustments | [2],[4] | (33) |  |
Prior Period Development, net Adjustments |  | 1 |  |
Net Prior Period Development |  | (92) | (212) | (252)
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 |  | (37) |  |
Overseas General Insurance | prior to 2010 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (17) |  |
Overseas General Insurance | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (43) |  |
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 |  | (68) |  |
Other PPD adjustments | [2] | (7) |  |
Prior Period Development, net Adjustments |  | 0 |  |
Net Prior Period Development |  | (68) | (67) | (71)
Overseas General Insurance | Long Tail [Member] | prior to 2010 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (18) |  |
Overseas General Insurance | Long Tail [Member] | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (43) |  |
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 |  | (25) |  |
Other PPD adjustments | [2] | (26) |  |
Prior Period Development, net Adjustments |  | 1 |  |
Net Prior Period Development |  | (24) | (145) | (181)
Overseas General Insurance | Short Tail [Member] | prior to 2010 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 1 |  |
Overseas General Insurance | Short Tail [Member] | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 0 |  |
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 |  | (28) |  |
Other PPD adjustments | [2] | 0 |  |
Prior Period Development, net Adjustments |  | (1) |  |
Net Prior Period Development |  | (29) | (50) | (59)
Segment Global Reinsurance [Member] | prior to 2010 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (54) |  |
Segment Global Reinsurance [Member] | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 26 |  |
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 |  | (59) |  |
Other PPD adjustments | [2] | (1) |  |
Prior Period Development, net Adjustments |  | 0 |  |
Net Prior Period Development |  | (59) | (69) | (68)
Segment Global Reinsurance [Member] | Long Tail [Member] | prior to 2010 [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] | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (8) |  |
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 |  | 31 |  |
Other PPD adjustments | [2] | 1 |  |
Prior Period Development, net Adjustments |  | (1) |  |
Net Prior Period Development |  | 30 | 19 | 9
Segment Global Reinsurance [Member] | Short Tail [Member] | prior to 2010 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (4) |  |
Segment Global Reinsurance [Member] | Short Tail [Member] | 2010 to 2018 [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 34 |  |
North America Agricultural Insurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (80) | (110) | (119)
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 |  | (103) |  |
Prior Period Development, net Adjustments |  | 23 |  |
Net Prior Period Development |  | (80) | (110) | (119)
Corporate Segment [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | 153 | 45 | 278
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 |  | 153 |  |
Prior Period Development, net Adjustments |  | 0 |  |
Net Prior Period Development |  | 153 | 45 | 278
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 $53 million, $85 million and $108 million for 2019, 2018, and 2017, respectively.
[2] | Other includes the impact of foreign exchange.
[3] | Includes favorable development of $82 million related to our Alternative Risk Solutions business (U.S. and Bermuda) and an adjustment to exclude $22 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 $37 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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