Unpaid losses and loss expenses Unpaid losses and loss expenses (Details) (Supplementary PPD Reconciliation) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	[1]	$ (981)	$ (937)	$ (1,204)
Prior Period Development, net Adjustments		85	108	69
Net Prior Period Development		(896)	(829)	(1,135)
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		(81)
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		42
Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(486)	(423)	(817)
Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(410)	(406)	(318)
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		(886)
Other PPD adjustments	[2]	(205)
Prior Period Development, net Adjustments		55
Net Prior Period Development		(831)
Segments included in loss triangles [Domain] | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(495)
Segments included in loss triangles [Domain] | prior to 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(186)
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		(653)
Other PPD adjustments	[2],[3]	(153)
Prior Period Development, net Adjustments		43
Net Prior Period Development		(610)	(746)	(778)
North America Commercial P&C Insurance | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(437)
North America Commercial P&C Insurance | prior to 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(63)
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		(425)
Other PPD adjustments	[2]	(149)
Prior Period Development, net Adjustments		30
Net Prior Period Development		(395)	(562)	(693)
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		(194)	(123)
North America Commercial P&C Insurance | Long Tail [Member] | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(214)
North America Commercial P&C Insurance | Long Tail [Member] | prior to 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(62)
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		(228)
Other PPD adjustments	[2]	(4)
Prior Period Development, net Adjustments		13
Net Prior Period Development		(215)	(184)	(85)
North America Commercial P&C Insurance | Short Tail [Member] | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(223)
North America Commercial P&C Insurance | Short Tail [Member] | prior to 2009 [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]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		41	69	27
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		40
Other PPD adjustments	[2]	(7)
Prior Period Development, net Adjustments		1
Net Prior Period Development		41	69	27
North America Personal P&C Insurance [Member] | Short Tail [Member] | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		54
North America Personal P&C Insurance [Member] | Short Tail [Member] | prior to 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(7)
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		(216)
Other PPD adjustments	[2],[4]	(43)
Prior Period Development, net Adjustments		4
Net Prior Period Development		(212)	(252)	(423)
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		(31)
Overseas General Insurance | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(107)
Overseas General Insurance | prior to 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(66)
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		(67)
Other PPD adjustments	[2]	(3)
Prior Period Development, net Adjustments		0
Net Prior Period Development		(67)	(71)	(236)
Overseas General Insurance | Long Tail [Member] | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(9)
Overseas General Insurance | Long Tail [Member] | prior to 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(55)
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		(149)
Other PPD adjustments	[2]	(40)
Prior Period Development, net Adjustments		4
Net Prior Period Development		(145)	(181)	(187)
Overseas General Insurance | Short Tail [Member] | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(98)
Overseas General Insurance | Short Tail [Member] | prior to 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(11)
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		(57)
Other PPD adjustments	[2]	(2)
Prior Period Development, net Adjustments		7
Net Prior Period Development		(50)	(59)	(78)
Segment Global Reinsurance [Member] | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(5)
Segment Global Reinsurance [Member] | prior to 2009 [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]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(74)
Other PPD adjustments	[2]	(1)
Prior Period Development, net Adjustments		5
Net Prior Period Development		(69)	(68)	(77)
Segment Global Reinsurance [Member] | Long Tail [Member] | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(25)
Segment Global Reinsurance [Member] | Long Tail [Member] | prior to 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(48)
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		17
Other PPD adjustments	[2]	(1)
Prior Period Development, net Adjustments		2
Net Prior Period Development		19	9	(1)
Segment Global Reinsurance [Member] | Short Tail [Member] | 2009 to 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		20
Segment Global Reinsurance [Member] | Short Tail [Member] | prior to 2009 [Member]
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 Agricultural Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(110)	(119)	(72)
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		(140)
Prior Period Development, net Adjustments		30
Net Prior Period Development		(110)	(119)	(72)
Corporate Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		45	278	189
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		45
Prior Period Development, net Adjustments		0
Net Prior Period Development		45	278	189
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 $85 million, $108 million and $69 million for 2018, 2017, and 2016, respectively.

[2] Other includes the impact of foreign exchange.

[3] Includes favorable development of $81 million related to our Alternative Risk Solutions business (U.S. and Bermuda) and an adjustment to exclude $42 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 $31 million related to International A&H business; the remaining difference relates to a number of other items, none of which are individually material.