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Unpaid losses and loss expenses Unpaid losses and loss expenses (narrative) (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2020 | Mar. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | $ (118) | $ (204)
Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (32) | (56)
Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (86) | (148)
North America Commercial P&C Insurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (105) | (131)
North America Commercial P&C Insurance [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (43) | (65)
North America Commercial P&C Insurance [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (62) | (66)
North America Commercial P&C Insurance [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 49 |
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 2013 to 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 Commercial P&C Insurance [Member] | Accident years 2014 - 2019 [Member] | General Liability [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 23 |
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 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] | North America Workers' Compensation [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (28) |
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] | Accident years 2018 - 2019 [Member] | Credit card enhancement coverages [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 20 |
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 | (36) |
North America Agricultural Insurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (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
North America Agricultural Insurance [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | $ (14) | $ (61)

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