Unpaid losses and loss expenses Unpaid losses and loss expenses (Details) (Supplementary PPD Reconciliation) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	[1]	$ (845)	$ (981)	$ (937)
Prior Period Development, net Adjustments		53	85	108
Net Prior Period Development		(792)	(896)	(829)
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		(82)
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		22
Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(642)	(486)	(423)
Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(150)	(410)	(406)
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		(895)
Other PPD adjustments	[2]	(156)
Prior Period Development, net Adjustments		30
Net Prior Period Development		(865)
Segments included in loss triangles [Domain] | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(215)
Segments included in loss triangles [Domain] | 2010 to 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(524)
North America Commercial P&C Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(683)
Other PPD adjustments	[2],[3]	(118)
Prior Period Development, net Adjustments		34
Net Prior Period Development		(649)	(610)	(746)
North America Commercial P&C Insurance | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(143)
North America Commercial P&C Insurance | 2010 to 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(422)
North America Commercial P&C Insurance | 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		(707)
Other PPD adjustments	[2]	(110)
Prior Period Development, net Adjustments		39
Net Prior Period Development		(668)	(395)	(562)
North America Commercial P&C Insurance | Long Tail [Member] | North America Workers' Compensation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(303)	(194)
North America Commercial P&C Insurance | Long Tail [Member] | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(137)
North America Commercial P&C Insurance | Long Tail [Member] | 2010 to 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(460)
North America Commercial P&C Insurance | 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
Other PPD adjustments	[2]	(8)
Prior Period Development, net Adjustments		(5)
Net Prior Period Development		19	(215)	(184)
North America Commercial P&C Insurance | Short Tail [Member] | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(6)
North America Commercial P&C Insurance | Short Tail [Member] | 2010 to 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		38
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		(95)	41	69
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		(91)
Other PPD adjustments	[2]	(5)
Prior Period Development, net Adjustments		(4)
Net Prior Period Development		(95)	41	69
North America Personal P&C Insurance [Member] | Short Tail [Member] | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(1)
North America Personal P&C Insurance [Member] | Short Tail [Member] | 2010 to 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(85)
Overseas General Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(93)
Other PPD adjustments	[2],[4]	(33)
Prior Period Development, net Adjustments		1
Net Prior Period Development		(92)	(212)	(252)
Overseas General Insurance | 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		(37)
Overseas General Insurance | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(17)
Overseas General Insurance | 2010 to 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(43)
Overseas General Insurance | 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		(68)
Other PPD adjustments	[2]	(7)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(68)	(67)	(71)
Overseas General Insurance | Long Tail [Member] | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(18)
Overseas General Insurance | Long Tail [Member] | 2010 to 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(43)
Overseas General Insurance | 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		(25)
Other PPD adjustments	[2]	(26)
Prior Period Development, net Adjustments		1
Net Prior Period Development		(24)	(145)	(181)
Overseas General Insurance | Short Tail [Member] | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		1
Overseas General Insurance | Short Tail [Member] | 2010 to 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		0
Segment 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		(28)
Other PPD adjustments	[2]	0
Prior Period Development, net Adjustments		(1)
Net Prior Period Development		(29)	(50)	(59)
Segment Global Reinsurance [Member] | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(54)
Segment Global Reinsurance [Member] | 2010 to 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		26
Segment 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		(59)
Other PPD adjustments	[2]	(1)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(59)	(69)	(68)
Segment Global Reinsurance [Member] | Long Tail [Member] | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(50)
Segment Global Reinsurance [Member] | Long Tail [Member] | 2010 to 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(8)
Segment 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		31
Other PPD adjustments	[2]	1
Prior Period Development, net Adjustments		(1)
Net Prior Period Development		30	19	9
Segment Global Reinsurance [Member] | Short Tail [Member] | prior to 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(4)
Segment Global Reinsurance [Member] | Short Tail [Member] | 2010 to 2018 [Member]
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
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(80)	(110)	(119)
North America Agricultural Insurance | 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 | 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		(103)
Prior Period Development, net Adjustments		23
Net Prior Period Development		(80)	(110)	(119)
Corporate Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		153	45	278
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		153
Prior Period Development, net Adjustments		0
Net Prior Period Development		153	45	278
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 $53 million, $85 million and $108 million for 2019, 2018, and 2017, respectively.

[2] Other includes the impact of foreign exchange.

[3] Includes favorable development of $82 million related to our Alternative Risk Solutions business (U.S. and Bermuda) and an adjustment to exclude $22 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 favorable development of $37 million related to International A&H business; the remaining difference relates to a number of other items, none of which are individually material.