Unpaid losses and loss expenses Unpaid losses and loss expenses (Supplementary PPD Reconciliation) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ (882)
Prior Period Development, net Adjustments		83	$ 232	$ 30
Net Prior Period Development		(773)	(876)	(926)
Prior Period Development, net adjustments except related to A&H		109
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		88
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		92
Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		148	58	(44)
Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(921)	(934)	(882)
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		(1,135)
Other PPD adjustments	[1]	(285)
Prior Period Development, net Adjustments		103
Net Prior Period Development		(1,032)
Segments included in loss triangles [Domain] | 2014 - 2022 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(735)
Segments included in loss triangles [Domain] | Accident years prior to 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(115)
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		(592)
Other PPD adjustments	[1],[2]	(222)
Prior Period Development, net Adjustments	[3]	98
Net Prior Period Development		(494)	(562)	(762)
North America Commercial P&C Insurance [Member] | 2014 - 2022 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(294)
North America Commercial P&C Insurance [Member] | Accident years prior to 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(76)
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		(176)
Other PPD adjustments	[1]	(187)
Prior Period Development, net Adjustments		90
Net Prior Period Development		(86)	(229)	(482)
North America Commercial P&C Insurance [Member] | Long Tail [Member] | 2014 - 2022 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		83
North America Commercial P&C Insurance [Member] | Long Tail [Member] | Accident years prior to 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(72)
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		(416)
Other PPD adjustments	[1]	(35)
Prior Period Development, net Adjustments		8
Net Prior Period Development		(408)	(333)	(280)
North America Commercial P&C Insurance [Member] | Short Tail [Member] | 2014 - 2022 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(377)
North America Commercial P&C Insurance [Member] | Short Tail [Member] | Accident years prior to 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(4)
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		(134)	(186)	(305)
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		(132)
Other PPD adjustments	[1]	(4)
Prior Period Development, net Adjustments		(2)
Net Prior Period Development		(134)	(186)	(305)
North America Personal P&C Insurance [Member] | Short Tail [Member] | 2014 - 2022 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(131)
North America Personal P&C Insurance [Member] | Short Tail [Member] | Accident years prior to 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		3
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		(376)
Other PPD adjustments	[1],[4]	(60)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(376)	(448)	(441)
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		56
Overseas General Insurance [Member] | 2014 - 2022 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)
Overseas General Insurance [Member] | Accident years prior to 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(26)
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		(50)
Other PPD adjustments	[1]	(2)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(50)	(65)	(106)
Overseas General Insurance [Member] | Long Tail [Member] | 2014 - 2022 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(26)
Overseas General Insurance [Member] | Long Tail [Member] | Accident years prior to 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(22)
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		(326)
Other PPD adjustments	[1]	(58)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(326)	(383)	(335)
Overseas General Insurance [Member] | Short Tail [Member] | 2014 - 2022 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(264)
Overseas General Insurance [Member] | Short Tail [Member] | Accident years prior to 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(4)
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		(35)
Other PPD adjustments	[1]	1
Prior Period Development, net Adjustments		7
Net Prior Period Development		(28)	22	3
Global Reinsurance [Member] | 2014 - 2022 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 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(16)
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		7
Other PPD adjustments	[1]	1
Prior Period Development, net Adjustments		0
Net Prior Period Development		7	(7)	(25)
Global Reinsurance [Member] | Long Tail [Member] | 2014 - 2022 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		22
Global Reinsurance [Member] | Long Tail [Member] | Accident years prior to 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(16)
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		(42)
Other PPD adjustments	[1]	0
Prior Period Development, net Adjustments		7
Net Prior Period Development		(35)	29	28
Global Reinsurance [Member] | Short Tail [Member] | 2014 - 2022 accident years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(42)
Global Reinsurance [Member] | Short Tail [Member] | Accident years prior to 2014
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		(18)	(61)	10
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		(24)
Prior Period Development, net Adjustments		6
Net Prior Period Development		(18)	(61)	10
Corporate Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		277	359	569
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		277
Prior Period Development, net Adjustments		0
Net Prior Period Development		277	359	569
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] Other includes the impact of foreign exchange.

[2] Includes favorable development of $88 million related to our Alternative Risk Solutions business (U.S. and Bermuda) and an adjustment to exclude $92 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.

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

[4] Includes favorable development of $56 million related to International A&H business; the remaining difference relates to a number of other items, none of which are individually material.