Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (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)
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(75)	(278)	(137)	(197)
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(246)	132	(644)	8
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 Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(101)	(255)	(243)	(439)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(56)	55	(197)	(12)
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			(255)	(337)
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
North America Commercial P&C Insurance [Member] | Multi-Line | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	46	26
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
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		29
North America Commercial P&C Insurance [Member] | Accident years 2016 and prior [Member] | Workers' Compensation Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(97)	(155)
North America Commercial P&C Insurance [Member] | Accident years 2016 and prior [Member] | Multi-Line | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(41)		(41)
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			74
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 2019 [Member] | Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		21
North America Commercial P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Property Insurance [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			(83)
North America Commercial P&C Insurance [Member] | Accident Years 2019 - 2020 | First Party Cyber risk [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 | Multi-Line | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(36)		(36)
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			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			74
North America Commercial P&C Insurance [Member] | Accident years 2018 and 2019 | 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 2019 and 2020 | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(54)
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Foreign Casualty Line | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(40)		(40)
North America Commercial P&C Insurance [Member] | Accident years 2014 and prior, and accident year 2019 | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(47)
North America Agricultural Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	7	18	5	4
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	7	18	5	4
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	43	54	140	339
Corporate Segment [Member] | Environmental Issue [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	33	35
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	43	54	140	339
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	(28)	(60)	(209)	(100)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(13)	(71)	(8)	(72)
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(15)	11	(201)	(28)
Overseas General Insurance [Member] | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		57
Overseas General Insurance [Member] | Casualty lines | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(72)	(94)
Overseas General Insurance [Member] | Accident years 2016 and prior [Member] | Casualty lines | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(143)
Overseas General Insurance [Member] | Accident Years 2016 - 2019 [Member] | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		125
Overseas General Insurance [Member] | Accident year 2017 - 2018 [Member] | Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(22)
Overseas General Insurance [Member] | Accident Years 2017 to 2019 [Member] | Casualty lines | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		49
Overseas General Insurance [Member] | Accident years 2018 - 2019 [Member] | Political Risk | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(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			(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			(54)
Overseas General Insurance [Member] | Accident years 2017 and prior | Casualty lines | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(97)
Overseas General Insurance [Member] | Accident years 2017 and prior | Environment and Political Risk | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(24)
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			(35)
Overseas General Insurance [Member] | Accident years 2018 - 2020 | Casualty lines | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	25
Overseas General Insurance [Member] | Accident Year 2015 and prior | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(68)
Overseas General Insurance [Member] | Accident years 2011 - 2020 | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	84
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 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	(182)	48	(266)	48
North America Personal P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Lines | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(22)
North America Personal P&C Insurance [Member] | Accident Years 2017 to 2019 [Member] | Homeowners and Valuables | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				69
North America Personal P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Auto Liability [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(43)
North America Personal P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Homeowners and Valuables | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(176)		(216)
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(4)	(6)	(11)	(29)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(4)	(6)	(26)	(25)
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ 0	$ 0	$ 15	$ (4)