Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)			$ 2,544
Net Prior Period Development	$ (192)	$ (200)	(399)	$ (396)
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(8)	(72)	96	(53)
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(184)	(128)	(495)	(343)
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(144)	(146)	(192)	(218)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(142)	(139)	(46)	(130)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(2)	(7)	(146)	(88)
North America Commercial P&C Insurance [Member] | Workers' Compensation Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(288)	(264)	(338)	(308)
North America Commercial P&C Insurance [Member] | Auto Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	117	119	167	125
North America Commercial P&C Insurance [Member] | Surety, Property and Marine Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(144)
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			97
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(61)	(61)	(150)	(204)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	36	0	35	0
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(97)	(61)	(185)	(204)
Overseas General Insurance [Member] | Segment Insurance Overseas General, Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(85)
Overseas General Insurance [Member] | Segment Insurance Overseas General, Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development			(73)
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		110
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	93	60	102	70
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	93	60	102	70
Corporate Segment [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ 0	$ 0	$ 0	$ 0