Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022	Dec. 31, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (196)	$ (240)
Unpaid losses and loss expenses	75,417	73,196	$ 75,747	$ 72,330
Net loss and loss expenses paid, Prior Years	3,911	3,569
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	19	(12)
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(215)	(228)
Boy Scouts of America Agreement in Principle [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss expenses	300		800
Net loss and loss expenses paid, Prior Years	200		$ 300
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(72)	(108)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	9	(20)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(81)	(88)
North America Commercial P&C Insurance [Member] | Accident and Health Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(76)
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		60
North America Agricultural Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	(26)
North America Agricultural Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0
North America Agricultural Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	(26)
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	10	8
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	10	8
Corporate Segment [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(143)	(60)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(143)	(60)
Overseas General Insurance [Member] | Property, Liability and Casualty Insurance Product Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(85)
Overseas General Insurance [Member] | Property, Liability and Casualty Insurance Product Line | Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(43)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	17	(51)
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
North America Personal P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	17	(51)
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(8)	(3)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	0	0
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (8)	$ (3)