Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (222)	$ (321)	$ (709)	$ (781)
Liability for Claims and Claims Adjustment Expense	75,992	72,631	75,992	72,631	$ 72,943	$ 67,811
Net loss and loss expenses paid, Prior Years			9,217	7,922
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	39	(75)	(55)	(137)
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(261)	(246)	(654)	(644)
Boy Scouts of America Agreement in Principle [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Claims and Claims Adjustment Expense	600		600		$ 800
Net loss and loss expenses paid, Prior Years	200
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(166)	(157)	(561)	(440)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(29)	(101)	(315)	(243)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(137)	(56)	(246)	(197)
North America Commercial P&C Insurance [Member] | Workers' Compensation Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(164)	(97)		(255)
North America Commercial P&C Insurance [Member] | Auto Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	58			74
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			74
North America Commercial P&C Insurance [Member] | Multi-Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(66)
North America Commercial P&C Insurance [Member] | Property and Inland Marine [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(132)
North America Agricultural Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	9	7	(17)	5
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	7	(17)	5
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	73	43	272	140
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			155	68
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(5)	(28)	(238)	(209)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(5)	(13)	(5)	(8)
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	(15)	(233)	(201)
Overseas General Insurance [Member] | Accident and Health Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(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)	(21)
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)
Overseas General Insurance [Member] | Environmental lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(34)
Overseas General Insurance [Member] | Casualty lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(82)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(133)	(182)	(187)	(266)
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	(133)	(182)	(187)	(266)
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	(4)	22	(11)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	(4)	(7)	(26)
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ 0	$ 0	$ 29	$ 15