Unpaid losses and loss expenses Unpaid losses and loss expenses (Supplementary PPD Reconciliation) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	[1]	$ (991)
Prior Period Development, net Adjustments		119	$ 83	$ 232
Net Prior Period Development		(856)	(773)	(876)
Prior Period Development, net adjustments except related to A&H		135
Alternative Risk Solutions [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		39
North America Workers' Compensation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		89
Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		288	148	58
Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(1,144)	(921)	(934)
Segments included in loss triangles [Domain]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(1,117)
Other PPD adjustments	[2]	(208)
Prior Period Development, net Adjustments		69
Net Prior Period Development		(1,048)
Segments included in loss triangles [Domain] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(887)
Segments included in loss triangles [Domain] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(22)
North America Commercial P&C Insurance [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(493)
Other PPD adjustments	[2],[3]	(200)
Prior Period Development, net Adjustments	[4]	65
Net Prior Period Development		(428)	(494)	(562)
North America Commercial P&C Insurance [Member] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(225)
North America Commercial P&C Insurance [Member] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(68)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(53)
Other PPD adjustments	[2]	(194)
Prior Period Development, net Adjustments		71
Net Prior Period Development		18	(86)	(229)
North America Commercial P&C Insurance [Member] | Long Tail [Member] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		227
North America Commercial P&C Insurance [Member] | Long Tail [Member] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(86)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(440)
Other PPD adjustments	[2]	(6)
Prior Period Development, net Adjustments		(6)
Net Prior Period Development		(446)	(408)	(333)
North America Commercial P&C Insurance [Member] | Short Tail [Member] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(452)
North America Commercial P&C Insurance [Member] | Short Tail [Member] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		18
North America Personal P&C Insurance [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(305)	(134)	(186)
North America Personal P&C Insurance [Member] | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		0	0	0
North America Personal P&C Insurance [Member] | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(305)
Other PPD adjustments	[2]	(4)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(305)	(134)	(186)
North America Personal P&C Insurance [Member] | Short Tail [Member] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(290)
North America Personal P&C Insurance [Member] | Short Tail [Member] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(11)
Overseas General Insurance [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(290)
Other PPD adjustments	[2]	(2)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(290)	(376)	(448)
Overseas General Insurance [Member] | International A&H [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		16
Overseas General Insurance [Member] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(352)
Overseas General Insurance [Member] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		64
Overseas General Insurance [Member] | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(26)
Other PPD adjustments	[2]	1
Prior Period Development, net Adjustments		0
Net Prior Period Development		(26)	(50)	(65)
Overseas General Insurance [Member] | Long Tail [Member] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(102)
Overseas General Insurance [Member] | Long Tail [Member] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		75
Overseas General Insurance [Member] | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(264)
Other PPD adjustments	[2]	(3)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(264)	(326)	(383)
Overseas General Insurance [Member] | Short Tail [Member] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(250)
Overseas General Insurance [Member] | Short Tail [Member] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(11)
Global Reinsurance [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(29)
Other PPD adjustments	[2]	(2)
Prior Period Development, net Adjustments		4
Net Prior Period Development		(25)	(28)	22
Global Reinsurance [Member] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(20)
Global Reinsurance [Member] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(7)
Global Reinsurance [Member] | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(1)
Other PPD adjustments	[2]	(2)
Prior Period Development, net Adjustments		1
Net Prior Period Development		0	7	(7)
Global Reinsurance [Member] | Long Tail [Member] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		8
Global Reinsurance [Member] | Long Tail [Member] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(7)
Global Reinsurance [Member] | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(28)
Other PPD adjustments	[2]	0
Prior Period Development, net Adjustments		3
Net Prior Period Development		(25)	(35)	29
Global Reinsurance [Member] | Short Tail [Member] | 2015 - 2023 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(28)
Global Reinsurance [Member] | Short Tail [Member] | Accident years prior to 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		0
North America Agricultural Insurance [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(104)	(18)	(61)
North America Agricultural Insurance [Member] | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		0	0	0
North America Agricultural Insurance [Member] | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(170)
Prior Period Development, net Adjustments		66
Net Prior Period Development		(104)	(18)	(61)
Corporate Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		296	277	359
Corporate Segment [Member] | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		296
Prior Period Development, net Adjustments		0
Net Prior Period Development		296	277	359
Corporate Segment [Member] | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 0	$ 0	$ 0

[1] Includes favorable development of $16 million related to long duration International A&H business.

[2] Other includes the impact of foreign exchange.

[3] Includes favorable development of $39 million related to our Alternative Risk Solutions business (U.S. and Bermuda) and an adjustment to exclude $89 million in unfavorable development in the workers' compensation line, associated with an increase in exposure for which additional premiums were collected; the remaining difference relates to a number of other items, none of which are individually material.

[4] Includes premium returns associated with our Alternative Risk Solutions business, which is excluded from the triangles.