Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)			$ 2,415
Net Prior Period Development	$ (249)	$ (192)	(504)	$ (399)
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	30	(8)	87	96
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(279)	(184)	(591)	(495)
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(106)	(144)	(220)	(192)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(75)	(142)	(27)	(46)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	(31)	(2)	(193)	(146)
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	(77)	(61)	(198)	(150)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	35	36	36	35
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development	$ (112)	$ (97)	$ (234)	$ (185)