Unpaid losses and loss expenses Unpaid losses and loss expenses (PPD table) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (200)	$ (222)	$ (596)	$ (709)
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(84)	(166)	(302)	(561)
North America Commercial P&C Insurance [Member] | Auto Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		58
North America Commercial P&C Insurance [Member] | Workers' Compensation Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	71	(164)	379
North America Commercial P&C Insurance [Member] | Commercial Excess and Umbrella [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		71
North America Commercial P&C Insurance [Member] | Property and Inland Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(132)
North America Commercial P&C Insurance [Member] | Multi-Line Loss Sensitive
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(66)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(119)	(133)	(135)	(187)
North American Agriculture Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(9)	9	(12)	(17)
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(49)	(5)	(253)	(238)
Overseas General Insurance [Member] | Accident and Health Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			45	(75)
Overseas General Insurance [Member] | Property, Liability and Casualty Insurance Product Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(67)
Overseas General Insurance [Member] | Property and Inland Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			147
Overseas General Insurance [Member] | Casualty lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(82)
Overseas General Insurance [Member] | Environmental lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(34)
Overseas General Insurance [Member] | Financial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		114
Overseas General Insurance [Member] | Personal Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development				(53)
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0	(25)	22
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		73	131	272
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(316)	(261)	(659)	(654)
Short Tail [Member] | North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(191)	(137)	(279)	(246)
Short Tail [Member] | North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(119)	(133)	(135)	(187)
Short Tail [Member] | North American Agriculture Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(9)	9	(12)	(17)
Short Tail [Member] | Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	3	0	(201)	(233)
Short Tail [Member] | Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0	(32)	29
Short Tail [Member] | Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0	0	0
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	116	39	63	(55)
Long Tail [Member] | North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	107	(29)	(23)	(315)
Long Tail [Member] | North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0	0	0
Long Tail [Member] | North American Agriculture Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0	0	0
Long Tail [Member] | Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(52)	(5)	(52)	(5)
Long Tail [Member] | Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0	7	(7)
Long Tail [Member] | Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ 61	$ 73	131	272
Long Tail [Member] | Corporate Segment [Member] | Other [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			$ 49	$ 155