Unpaid losses and loss expenses Unpaid losses and loss expenses (Details) (Supplementary PPD Reconciliation) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	[1]	$ (986)	$ (414)	$ (845)
Prior Period Development, net Adjustments		60	19	53
Net Prior Period Development		(926)	(395)	(792)
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		(96)
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		(48)
Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(44)	(313)	(642)
Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(882)	(82)	(150)
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		(1,593)
Other PPD adjustments	[2]	(296)
Prior Period Development, net Adjustments		88
Net Prior Period Development		(1,505)
Segments included in loss triangles [Domain] | prior to 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(309)
Segments included in loss triangles [Domain] | 2012 - 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(988)
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		(841)
Other PPD adjustments	[2],[3]	(152)
Prior Period Development, net Adjustments	[4]	79
Net Prior Period Development		(762)	(702)	(649)
North America Commercial P&C Insurance [Member] | prior to 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(194)
North America Commercial P&C Insurance [Member] | 2012 - 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(495)
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		(551)
Other PPD adjustments	[2]	(142)
Prior Period Development, net Adjustments		69
Net Prior Period Development		(482)	(672)	(668)
North America Commercial P&C Insurance [Member] | Long Tail [Member] | prior to 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(191)
North America Commercial P&C Insurance [Member] | Long Tail [Member] | 2012 - 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(218)
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		(290)
Other PPD adjustments	[2]	(10)
Prior Period Development, net Adjustments		10
Net Prior Period Development		(280)	(30)	19
North America Commercial P&C Insurance [Member] | Short Tail [Member] | prior to 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(3)
North America Commercial P&C Insurance [Member] | Short Tail [Member] | 2012 - 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(277)
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)	63	(95)
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		(304)
Other PPD adjustments	[2]	(9)
Prior Period Development, net Adjustments		(1)
Net Prior Period Development		(305)	63	(95)
North America Personal P&C Insurance [Member] | Short Tail [Member] | prior to 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(2)
North America Personal P&C Insurance [Member] | Short Tail [Member] | 2012 - 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(293)
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],[5]	(134)
Prior Period Development, net Adjustments		7
Net Prior Period Development		(441)	(150)	(92)
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		(118)
Overseas General Insurance [Member] | prior to 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(82)
Overseas General Insurance [Member] | 2012 - 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(232)
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		(106)
Other PPD adjustments	[2]	(8)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(106)	(49)	(68)
Overseas General Insurance [Member] | Long Tail [Member] | prior to 2012
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] | Long Tail [Member] | 2012 - 2020
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		(342)
Other PPD adjustments	[2]	(126)
Prior Period Development, net Adjustments		7
Net Prior Period Development		(335)	(101)	(24)
Overseas General Insurance [Member] | Short Tail [Member] | prior to 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(6)
Overseas General Insurance [Member] | Short Tail [Member] | 2012 - 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(210)
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		0
Other PPD adjustments	[2]	(1)
Prior Period Development, net Adjustments		3
Net Prior Period Development		3	(29)	(29)
Global Reinsurance [Member] | prior to 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(31)
Global Reinsurance [Member] | 2012 - 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		32
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		(28)
Other PPD adjustments	[2]	1
Prior Period Development, net Adjustments		3
Net Prior Period Development		(25)	(25)	(59)
Global Reinsurance [Member] | Long Tail [Member] | prior to 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(27)
Global Reinsurance [Member] | Long Tail [Member] | 2012 - 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(2)
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]	(2)
Prior Period Development, net Adjustments		0
Net Prior Period Development		28	(4)	30
Global Reinsurance [Member] | Short Tail [Member] | prior to 2012
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] | 2012 - 2020
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		10	(10)	(80)
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		38
Prior Period Development, net Adjustments		(28)
Net Prior Period Development		10	(10)	(80)
Corporate Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		569	433	153
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		569
Prior Period Development, net Adjustments		0
Net Prior Period Development		569	433	153
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 $60 million, $19 million, and $53 million for 2021, 2020, and 2019, respectively.

[2] Other includes the impact of foreign exchange.

[3] Includes favorable development of $96 million related to our Alternative Risk Solutions business (U.S. and Bermuda) and an adjustment to exclude $48 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 favorable development of $118 million related to International A&H business; the remaining difference relates to a number of other items, none of which are individually material.