Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (511)	$ (361)	$ (1,080)	$ (865)
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	64	99	210	186
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(575)	(460)	(1,290)	(1,051)
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(14)	(26)	(214)	(246)
North America Commercial P&C Insurance [Member] \| Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	192	104	144	77
North America Commercial P&C Insurance [Member] \| Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(206)	(130)	(358)	(323)
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(162)	(84)	(439)	(282)
Overseas General Insurance [Member] \| Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(174)	(66)	(163)	(30)
Overseas General Insurance [Member] \| Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ 12	$ (18)	$ (276)	$ (252)