Unpaid losses and loss expenses Unpaid losses and loss expenses (Supplementary PPD Reconciliation) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	[1]	$ (1,153)	$ (986)	$ (414)
Prior Period Development, net Adjustments		277	60	19
Net Prior Period Development		(876)	(926)	(395)
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		161
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		46
Colorado Wildfire
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		48
Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		58	(44)	(313)
Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(934)	(882)	(82)
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		(1,285)
Other PPD adjustments	[2]	(440)
Prior Period Development, net Adjustments		111
Net Prior Period Development		(1,174)
Segments included in loss triangles [Domain] | Accident years prior to 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(7)
Segments included in loss triangles [Domain] | 2013 - 2021 accident years (implied PPD per loss triangles)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(838)
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		(674)
Other PPD adjustments	[2],[3]	(283)
Prior Period Development, net Adjustments	[4]	112
Net Prior Period Development		(562)	(762)	(702)
North America Commercial P&C Insurance [Member] | Accident years prior to 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		11
North America Commercial P&C Insurance [Member] | 2013 - 2021 accident years (implied PPD per loss triangles)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(402)
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		(314)
Other PPD adjustments	[2]	(253)
Prior Period Development, net Adjustments		85
Net Prior Period Development		(229)	(482)	(672)
North America Commercial P&C Insurance [Member] | Long Tail [Member] | Accident years prior to 2013
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] | Long Tail [Member] | 2013 - 2021 accident years (implied PPD per loss triangles)
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] | 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		(360)
Other PPD adjustments	[2]	(30)
Prior Period Development, net Adjustments		27
Net Prior Period Development		(333)	(280)	(30)
North America Commercial P&C Insurance [Member] | Short Tail [Member] | Accident years prior to 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(11)
North America Commercial P&C Insurance [Member] | Short Tail [Member] | 2013 - 2021 accident years (implied PPD per loss triangles)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(319)
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		(186)	(305)	63
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		(186)
Other PPD adjustments	[2],[5]	(58)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(186)	(305)	63
North America Personal P&C Insurance [Member] | Short Tail [Member] | Accident years prior to 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(5)
North America Personal P&C Insurance [Member] | Short Tail [Member] | 2013 - 2021 accident years (implied PPD per loss triangles)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(123)
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		(448)
Other PPD adjustments	[2],[6]	(100)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(448)	(441)	(150)
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		105
Overseas General Insurance [Member] | Accident years prior to 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(2)
Overseas General Insurance [Member] | 2013 - 2021 accident years (implied PPD per loss triangles)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(346)
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		(65)
Other PPD adjustments	[2]	3
Prior Period Development, net Adjustments		0
Net Prior Period Development		(65)	(106)	(49)
Overseas General Insurance [Member] | Long Tail [Member] | Accident years prior to 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		8
Overseas General Insurance [Member] | Long Tail [Member] | 2013 - 2021 accident years (implied PPD per loss triangles)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(76)
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		(383)
Other PPD adjustments	[2]	(103)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(383)	(335)	(101)
Overseas General Insurance [Member] | Short Tail [Member] | Accident years prior to 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(10)
Overseas General Insurance [Member] | Short Tail [Member] | 2013 - 2021 accident years (implied PPD per loss triangles)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(270)
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		23
Other PPD adjustments	[2]	1
Prior Period Development, net Adjustments		(1)
Net Prior Period Development		22	3	(29)
Global Reinsurance [Member] | Accident years prior to 2013
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] | 2013 - 2021 accident years (implied PPD per loss triangles)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		33
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	[2]	1
Prior Period Development, net Adjustments		0
Net Prior Period Development		(7)	(25)	(25)
Global Reinsurance [Member] | Long Tail [Member] | Accident years prior to 2013
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] | 2013 - 2021 accident years (implied PPD per loss triangles)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(1)
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		30
Other PPD adjustments	[2]	0
Prior Period Development, net Adjustments		(1)
Net Prior Period Development		29	28	(4)
Global Reinsurance [Member] | Short Tail [Member] | Accident years prior to 2013
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] | Short Tail [Member] | 2013 - 2021 accident years (implied PPD per loss triangles)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		34
North America Agricultural Insurance [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(61)	10	(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		(227)
Prior Period Development, net Adjustments		166
Net Prior Period Development		(61)	10	(10)
Corporate Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		359	569	433
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		359
Prior Period Development, net Adjustments		0
Net Prior Period Development		359	569	433
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] Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments, and earned premiums totaling $277 million, $60 million, and $19 million for 2022, 2021, and 2020, respectively.

[2] Other includes the impact of foreign exchange.

[3] Includes favorable development of $161 million related to our Alternative Risk Solutions business (U.S. and Bermuda) and an adjustment to exclude $46 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.

[5] Includes $48 million relating to the Colorado Wildfire CAT event that began December 30, 2021.

[6] Includes favorable development of $105 million related to International A&H business; the remaining difference relates to a number of other items, none of which are individually material.