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Unpaid losses and loss expenses Unpaid losses and loss expenses (narrative) (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Liability for unpaid claims and claims adjustment expense, period increase (decrease), excluding CATs, PPD, and other | $ 557 |
Net Prior Period Development | (204) | $ (209)
Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (56) | 18
Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (148) | (227)
North America Commercial P&C Insurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (131) | (101)
North America Commercial P&C Insurance [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (65) | 8
North America Commercial P&C Insurance [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (66) | (109)
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 |  | 16
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 | 3 | 40
North America Commercial P&C Insurance [Member] | Other [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development |  | (34)
North America Commercial P&C Insurance [Member] | Surety Product Line [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development |  | (19)
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 years 2012 and prior [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development |  | (29)
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 2017 [Member] | Catastrophe [Member] | Other [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development |  | (3)
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Commercial Property and Marine [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development |  | (75)
North America Commercial P&C Insurance [Member] | Accident Year 2015 - 2017 [Member] | Other [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (17) |
North America Commercial P&C Insurance [Member] | Accident year 2015 and prior [Member] | Professional Malpractice Liability Insurance [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (57) |
North America Commercial P&C Insurance [Member] | Accident year 2015 and prior [Member] | North America Workers' Compensation [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (30) |
North America Commercial P&C Insurance [Member] | Accident years 2013 - 2018 [Member] | Auto & General Liability [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 50 |
North America Personal P&C Insurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (10) | (6)
North America Personal P&C Insurance [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 0 | 0
North America Personal P&C Insurance [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (10) | (6)
North America Personal P&C Insurance [Member] | Catastrophe [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development |  | (6)
North America Personal P&C Insurance [Member] | Non-catastrophe [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 122 |
North America Personal P&C Insurance [Member] | Accident years 2017 - 2018 [Member] | Catastrophe [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (132) |
North America Agricultural Insurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (61) | (76)
North America Agricultural Insurance [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 0 | 0
North America Agricultural Insurance [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (61) | (76)
Overseas General Insurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (4) | (22)
Overseas General Insurance [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 0 | 0
Overseas General Insurance [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (4) | (22)
Overseas General Insurance [Member] | Other [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (7) |
Overseas General Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development |  | (12)
Overseas General Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Catastrophe [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 3 |
Global Reinsurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (8) | (14)
Global Reinsurance [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (1) | 0
Global Reinsurance [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (7) | (14)
Global Reinsurance [Member] | Other [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (12) |
Global Reinsurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development |  | (10)
Global Reinsurance [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Catastrophe [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 5 |
Corporate Segment [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 10 | 10
Corporate Segment [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 10 | 10
Corporate Segment [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | $ 0 | $ 0

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