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Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | $ 75 | $ (188) | $ (43) | $ (392)
Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 113 | (206) | 81 | (262)
Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (38) | 18 | (124) | (130)
North America Commercial P&C Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (146) | (185) | (251) | (316)
North America Commercial P&C Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (141) | (206) | (184) | (271)
North America Commercial P&C Insurance [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (5) | 21 | (67) | (45)
North America Commercial P&C Insurance [Member] | Professional Malpractice Liability Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  |  | (54)
North America Commercial P&C Insurance [Member] | General Liability [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | (36) |
North America Commercial P&C Insurance [Member] | Workers' Compensation Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (152) | (163) | (182) |
North America Commercial P&C Insurance [Member] | Other [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 5 |  | 5 |
North America Commercial P&C Insurance [Member] | Accident Year 2013 and prior [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  |  | (31)
North America Commercial P&C Insurance [Member] | Accident year 2015 and prior [Member] | General Liability [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (50) |  |
North America Commercial P&C Insurance [Member] | Accident years 2016 and prior [Member] | Professional Malpractice Liability Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | (66) |
North America Commercial P&C Insurance [Member] | Accident years 2016 and prior [Member] | Environmental lines [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | (43) |
North America Commercial P&C Insurance [Member] | Accident years 2016 and prior [Member] | General Liability [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (59) |  |  |
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Surety Product Line [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  |  | (49)
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Surety Product Line [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | (31) |
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Workers' Compensation Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | (61) |  | (200)
North America Commercial P&C Insurance [Member] | Accident years 2018 - 2019 [Member] | Accident and Health Insurance Product Line [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | (37) |
North America Commercial P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Workers' Compensation Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (62) |  |  |
North America Commercial P&C Insurance [Member] | Accident years 2015 to 2019 [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | 40 |
North America Commercial P&C Insurance [Member] | Accident years 2015 to 2019 [Member] | Other [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | 75 |
North America Commercial P&C Insurance [Member] | Accident years 2014 - 2018 [Member] | Other [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  | 25 |  | 31
North America Agricultural Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 0 | 0 | (14) | (61)
North America Agricultural Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 0 | 0 | 0 | 0
North America Agricultural Insurance [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 0 | 0 | (14) | (61)
Corporate Segment [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 274 | 33 | 285 | 43
Corporate Segment [Member] | Discontinued Operations [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 20 |  | 31 |
Corporate Segment [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 274 | 33 | 285 | 43
Corporate Segment [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 0 | 0 | 0 | 0
Corporate Segment [Member] | Other [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | 254 |
Overseas General Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (36) | (20) | (40) | (24)
Overseas General Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (1) | 0 | (1) | 0
Overseas General Insurance [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (35) | $ (20) | (39) | $ (24)
Overseas General Insurance [Member] | Marine [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | $ (19) |  | $ (22) |

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