Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (283)	$ (249)	$ (569)	$ (504)
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	110	30	146	87
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(393)	(279)	(715)	(591)
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(111)	(106)	(200)	(220)
North America Commercial P&C Insurance [Member] \| Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(69)	(75)	(48)	(27)
North America Commercial P&C Insurance [Member] \| Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(42)	(31)	(152)	(193)
North America Commercial P&C Insurance [Member] \| Risk Management Business
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(163)
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(146)	(77)	(277)	(198)
Overseas General Insurance [Member] \| Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	11	35	11	36
Overseas General Insurance [Member] \| Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (157)	$ (112)	$ (288)	$ (234)