Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (268)	$ 75	$ (460)	$ (43)
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(25)	113	(62)	81
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(243)	(38)	(398)	(124)
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(156)	(146)	(283)	(251)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(96)	(141)	(142)	(184)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(60)	(5)	(141)	(67)
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] | Surety Product Line [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] | Workers' Compensation Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(137)	(152)	(158)	(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] | Auto Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			74
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] | Auto Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		75		75
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] | Accident Years 2016 - 2019 [Member] | Commercial Excess and Umbrella [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 year 2017 and prior [Member] | General Liability [Member] | Long 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 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] | 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] | Short-Duration Insurance Contract, Accident Year 2020 | Surety Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(35)
North America Commercial P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Workers' Compensation Insurance [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] | Short-Duration Insurance Contract, Accident Year 2020 | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(29)
North America Commercial P&C Insurance [Member] | Accident Years 2019 - 2020 | Professional Malpractice Liability Insurance [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] | Accident Years 2019 - 2020 | Accident and Health Insurance Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(46)
North America Commercial P&C Insurance [Member] | Accident years 2018 - 2020 [Member] | Surety Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(89)
North America Commercial P&C Insurance [Member] | Accident years 2017 and prior | Environmental lines [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 2017 and prior | General Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(45)
North America Commercial P&C Insurance [Member] | Accident years 2016 to 2020 | General Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	27		26
North America Commercial P&C Insurance [Member] | Accident years 2016 and 2017 | Professional Malpractice Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(51)
North America Commercial P&C Insurance [Member] | Accident years 2017 and 2019 | Auto Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	76
North America Agricultural Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0	(2)	(14)
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	(2)	(14)
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	88	274	97	285
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	88	274	97	285
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			68	254
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(156)	(36)	(181)	(40)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	5	(1)	5	(1)
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(161)	(35)	(186)	(39)
Overseas General Insurance [Member] | Marine [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(19)		(22)
Overseas General Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Property, Liability and Casualty Insurance Product Line | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(21)
Overseas General Insurance [Member] | Accident Years 2019 - 2020 | Accident and Health Insurance Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(49)		(55)
Overseas General Insurance [Member] | Accident Years 2019 - 2020 | Property, Liability and Casualty Insurance Product Line | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(50)		(58)
Overseas General Insurance [Member] | Accident years 2018 - 2020 | Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(31)		(35)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(44)	(1)	(84)	0
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	(44)	(1)	(84)	0
North America Personal P&C Insurance [Member] | Auto Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(43)		(43)
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	(16)	(7)	(23)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(22)	(19)	(22)	(19)
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ 22	$ 3	$ 15	$ (4)