Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jul. 01, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (200)	$ (222)	$ (596)	$ (709)
Unpaid losses and loss expenses	79,705	75,336	79,705	75,336		$ 75,747	$ 72,330
Net loss and loss expenses paid, Prior Years			10,158	9,217
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease), Gross			4,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	61,897	58,023	61,897	58,023		$ 58,661	56,198
Huatai Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss expenses					$ 831
Liability for Unpaid Claims and Claims Adjustment Expense, Net					$ 405		$ 0
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	116	39	63	(55)
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(316)	(261)	(659)	(654)
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] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	107	(29)	(23)	(315)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(191)	(137)	(279)	(246)
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] | 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] | 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] | Auto Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		58
North America Agricultural Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(9)	9	(12)	(17)
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	(9)	9	(12)	(17)
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		73	131	272
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	61	73	131	272
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] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			49	155
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] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(52)	(5)	(52)	(5)
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	3	0	(201)	(233)
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] | financial lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	77
Overseas General Insurance [Member] | Political Risk
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	51
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 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	(119)	(133)	(135)	(187)
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0	(25)	22
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0	7	(7)
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	$ 0	(32)	$ 29
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	222		227
North America Commercial P&C Insurance [Member] | Property and Inland Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	188		113
North America Commercial P&C Insurance [Member] | Surety Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			101
North America Commercial P&C Insurance [Member] | Auto Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			125
North American Personal P&C Insurance [Member] | Personal Excess Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	145
North American Personal P&C Insurance [Member] | homeowners and valuables lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ 263		$ 244