Unpaid losses and loss expenses Unpaid losses and loss expenses (narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (167)	$ (243)	$ (559)	$ (643)
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(252)	(246)	(514)	(319)
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	85	3	(45)	(324)
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(109)	(216)	(425)	(472)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(197)	(170)	(468)	(266)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	88	(46)	43	(206)
North America Commercial P&C Insurance [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(294)	(183)
North America Commercial P&C Insurance [Member] | Foreign Casualty Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(39)	(28)
North America Commercial P&C Insurance [Member] | Multi-Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(34)		(36)
North America Commercial P&C Insurance [Member] | Auto Liability Excess Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			31	71
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			(63)
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				(50)
North America Commercial P&C Insurance [Member] | Commercial Property and Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(41)		(156)
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] | Catastrophe [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(27)
North America Commercial P&C Insurance [Member] | Non-catastrophe [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	116
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	(32)
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		(93)		(123)
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(56)
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] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(42)		(157)
North America Commercial P&C Insurance [Member] | Accident years 2014 and prior [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(45)		(127)
North America Commercial P&C Insurance [Member] | Accident years 2014 and prior [Member] | Foreign Casualty Line [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] | Accident years 2017 - 2018 [Member] | Catastrophe [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(41)
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(61)
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	154
North America Commercial P&C Insurance [Member] | Accident years 2017 and prior [Member] | Non-catastrophe [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(38)
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			(53)
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	(94)
North America Commercial P&C Insurance [Member] | Accident year 2015 and prior [Member] | Foreign Casualty Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(39)
North America Commercial P&C Insurance [Member] | Accident year 2015 and prior [Member] | Multi-Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(36)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(62)	58	(88)	59
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	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	(62)	58	(88)	59
North America Personal P&C Insurance [Member] | Auto Liability and Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(16)
North America Personal P&C Insurance [Member] | Non-catastrophe [Member] | Homeowners and Valuables [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		137
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Homeowners and Valuables [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		80
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(68)
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Homeowners and Valuables [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(57)
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal lines [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(24)
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 Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Homeowners and Valuables [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(36)		82
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal lines [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(26)		(26)
North America Agricultural Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	18	(1)	(43)	(77)
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	18	(1)	(43)	(77)
North America Agricultural Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(1)
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(25)	(72)	(49)	(166)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(66)	(49)	(66)	(51)
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	41	(23)	17	(115)
Overseas General Insurance [Member] | Political Risk [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(12)
Overseas General Insurance [Member] | Aviation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(10)
Overseas General Insurance [Member] | Personal lines [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(17)
Overseas General Insurance [Member] | Financial | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	50	38
Overseas General Insurance [Member] | Other [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(33)
Overseas General Insurance [Member] | Other [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(57)
Overseas General Insurance [Member] | Surety Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(17)
Overseas General Insurance [Member] | Accident and Health Insurance Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(16)
Overseas General Insurance [Member] | Energy Lines [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(13)
Overseas General Insurance [Member] | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(101)	(54)
Overseas General Insurance [Member] | Environmental Issue [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(10)
Overseas General Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		4		(8)
Overseas General Insurance [Member] | Accident years 2014 and prior [Member] | Financial | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(93)
Overseas General Insurance [Member] | Accident years 2014 and prior [Member] | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(92)
Overseas General Insurance [Member] | Accident years 2015 - 2017 [Member] | Financial | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		131
Overseas General Insurance [Member] | Accident years 2015 - 2017 [Member] | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		38
Overseas General Insurance [Member] | Accident years 2012 - 2015 [Member] | Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(21)
Overseas General Insurance [Member] | Accident years 2017 - 2018 [Member] | Surety Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	27
Overseas General Insurance [Member] | Accident years 2014 and 2016 [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(50)
Overseas General Insurance [Member] | Accident year 2015 and prior [Member] | Financial | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(75)
Overseas General Insurance [Member] | Accident year 2015 and prior [Member] | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(123)
Overseas General Insurance [Member] | Accident years 2016 - 2018 [Member] | Financial | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	125
Overseas General Insurance [Member] | Accident years 2016 - 2018 [Member] | Casualty Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	22
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(25)	(24)	(33)	(54)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(25)	(39)	(59)	(69)
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	15	26	15
Global Reinsurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		15		14
Global Reinsurance [Member] | Accident years 2017 - 2018 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			37
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	36	12	79	67
Corporate Segment [Member] | Discontinued Operations [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	9		28
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	36	12	79	67
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] | Environmental Issue [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ 27	$ 54