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Unpaid losses and loss expenses Unpaid losses and loss expenses (PPD table) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | $ (321) | $ (146) | $ (781) | $ (189)
North America Commercial P&C Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (157) | (200) | (440) | (451)
North America Personal P&C Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (182) | 48 | (266) | 48
North American Agriculture Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 7 | 18 | 5 | 4
Overseas General Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (28) | (60) | (209) | (100)
Global Reinsurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (4) | (6) | (11) | (29)
Corporate Segment [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 43 | 54 | 140 | 339
Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (246) | 132 | (644) | 8
Short Tail [Member] | North America Commercial P&C Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (56) | 55 | (197) | (12)
Short Tail [Member] | North America Personal P&C Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (182) | 48 | (266) | 48
Short Tail [Member] | North American Agriculture Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 7 | 18 | 5 | 4
Short Tail [Member] | Overseas General Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (15) | 11 | (201) | (28)
Short Tail [Member] | Global Reinsurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 0 | 0 | 15 | (4)
Short Tail [Member] | Corporate Segment [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 0 | 0 | 0 | 0
Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (75) | (278) | (137) | (197)
Long Tail [Member] | North America Commercial P&C Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (101) | (255) | (243) | (439)
Long Tail [Member] | North America Personal P&C Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 0 | 0 | 0 | 0
Long Tail [Member] | North American Agriculture Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 0 | 0 | 0 | 0
Long Tail [Member] | Overseas General Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (13) | (71) | (8) | (72)
Long Tail [Member] | Global Reinsurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (4) | (6) | (26) | (25)
Long Tail [Member] | Corporate Segment [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | $ 43 | $ 54 | $ 140 | $ 339

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