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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, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense | [1] | $ (414) | $ (845) | $ (981)
Prior Period Development, net Adjustments |  | 19 | 53 | 85
Net Prior Period Development |  | (395) | (792) | (896)
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 |  | 80 |  |
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 |  | 15 |  |
Long Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (313) | (642) | (486)
Short Tail [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (82) | (150) | (410)
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 |  | (828) |  |
Other PPD adjustments | [2] | (125) |  |
Prior Period Development, net Adjustments |  | 10 |  |
Net Prior Period Development |  | (818) |  |
Segments included in loss triangles [Domain] | 2011 - 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (380) |  |
Segments included in loss triangles [Domain] | prior to 2011 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (323) |  |
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 |  | (733) |  |
Other PPD adjustments | [2],[3] | (97) |  |
Prior Period Development, net Adjustments |  | 31 |  |
Net Prior Period Development |  | (702) | (649) | (610)
North America Commercial P&C Insurance | 2011 - 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (391) |  |
North America Commercial P&C Insurance | prior to 2011 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (245) |  |
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 |  | (703) |  |
Other PPD adjustments | [2] | (102) |  |
Prior Period Development, net Adjustments |  | 31 |  |
Net Prior Period Development |  | (672) | (668) | (395)
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) |
North America Commercial P&C Insurance | Long Tail [Member] | 2011 - 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (354) |  |
North America Commercial P&C Insurance | Long Tail [Member] | prior to 2011 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (247) |  |
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 |  | (30) |  |
Other PPD adjustments | [2] | 5 |  |
Prior Period Development, net Adjustments |  | 0 |  |
Net Prior Period Development |  | (30) | 19 | (215)
North America Commercial P&C Insurance | Short Tail [Member] | 2011 - 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (37) |  |
North America Commercial P&C Insurance | Short Tail [Member] | prior to 2011 |  |  |  |
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 Personal P&C Insurance [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | 63 | (95) | 41
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 |  | 81 |  |
Other PPD adjustments | [2] | (3) |  |
Prior Period Development, net Adjustments |  | (18) |  |
Net Prior Period Development |  | 63 | (95) | 41
North America Personal P&C Insurance [Member] | Short Tail [Member] | 2011 - 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 85 |  |
North America Personal P&C Insurance [Member] | Short Tail [Member] | prior to 2011 |  |  |  |
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 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (150) |  |
Other PPD adjustments | [2],[4] | (23) |  |
Prior Period Development, net Adjustments |  | 0 |  |
Net Prior Period Development |  | (150) | (92) | (212)
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 |  | 22 |  |
Overseas General Insurance | 2011 - 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (60) |  |
Overseas General Insurance | prior to 2011 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (67) |  |
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 |  | (49) |  |
Other PPD adjustments | [2] | 1 |  |
Prior Period Development, net Adjustments |  | 0 |  |
Net Prior Period Development |  | (49) | (68) | (67)
Overseas General Insurance | Long Tail [Member] | 2011 - 2019 |  |  |  |
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 2011 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (59) |  |
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 |  | (101) |  |
Other PPD adjustments | [2] | (24) |  |
Prior Period Development, net Adjustments |  | 0 |  |
Net Prior Period Development |  | (101) | (24) | (145)
Overseas General Insurance | Short Tail [Member] | 2011 - 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (69) |  |
Overseas General Insurance | Short Tail [Member] | prior to 2011 |  |  |  |
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] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (26) |  |
Other PPD adjustments | [2] | (2) |  |
Prior Period Development, net Adjustments |  | (3) |  |
Net Prior Period Development |  | (29) | (29) | (50)
Segment Global Reinsurance [Member] | 2011 - 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (14) |  |
Segment Global Reinsurance [Member] | prior to 2011 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (10) |  |
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 |  | (23) |  |
Other PPD adjustments | [2] | (1) |  |
Prior Period Development, net Adjustments |  | (2) |  |
Net Prior Period Development |  | (25) | (59) | (69)
Segment Global Reinsurance [Member] | Long Tail [Member] | 2011 - 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (12) |  |
Segment Global Reinsurance [Member] | Long Tail [Member] | prior to 2011 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (10) |  |
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 |  | (3) |  |
Other PPD adjustments | [2] | (1) |  |
Prior Period Development, net Adjustments |  | (1) |  |
Net Prior Period Development |  | (4) | 30 | 19
Segment Global Reinsurance [Member] | Short Tail [Member] | 2011 - 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (2) |  |
Segment Global Reinsurance [Member] | Short Tail [Member] | prior to 2011 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 0 |  |
North America Agricultural Insurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (10) | (80) | (110)
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 |  | (19) |  |
Prior Period Development, net Adjustments |  | 9 |  |
Net Prior Period Development |  | (10) | (80) | (110)
Corporate Segment [Member] |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | 433 | 153 | 45
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 |  | 433 |  |
Prior Period Development, net Adjustments |  | 0 |  |
Net Prior Period Development |  | 433 | 153 | 45
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 $19 million, $53 million, and $85 million for 2020, 2019, and 2018, respectively.
[2] | Other includes the impact of foreign exchange.
[3] | Includes favorable development of $80 million related to our Alternative Risk Solutions business (U.S. and Bermuda) and an adjustment to exclude $15 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.(3) Includes premium returns associated with our Alternative Risk Solutions business, which is excluded from the triangles.
[4] | Includes favorable development of $22 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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