Unpaid losses and loss expenses Unpaid losses and loss expenses (Details) (Supplementary PPD Reconciliation) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
supplementary PPD reconciliation [Line Items]
PPD, Gross	[1]	$ (937)	$ (1,204)	$ (546)
prior period development, net adjustments		108	69	0
Prior Year Claims and Claims Adjustment Expense		(829)	(1,135)	(546)
Long Tail [Member]
supplementary PPD reconciliation [Line Items]
Prior Year Claims and Claims Adjustment Expense		(423)	(817)	(263)
Short Tail [Member]
supplementary PPD reconciliation [Line Items]
Prior Year Claims and Claims Adjustment Expense		(406)	(318)	(283)
Segments included in loss triangles [Domain]
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2]	(122)
PPD, Gross		(1,041)
prior period development, net adjustments		53
Prior Year Claims and Claims Adjustment Expense		(988)
Segments included in loss triangles [Domain] | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(658)
Segments included in loss triangles [Domain] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(261)
North America Commercial P&C Insurance
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2],[3]	(73)
PPD, Gross		(803)
prior period development, net adjustments		57
Prior Year Claims and Claims Adjustment Expense		(746)	(778)	(264)
North America Commercial P&C Insurance | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(555)
North America Commercial P&C Insurance | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(175)
North America Commercial P&C Insurance | Long Tail [Member]
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2]	(76)
PPD, Gross		(618)
prior period development, net adjustments		56
Prior Year Claims and Claims Adjustment Expense		(562)	(693)	(162)
North America Commercial P&C Insurance | Long Tail [Member] | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(367)
North America Commercial P&C Insurance | Long Tail [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(175)
North America Commercial P&C Insurance | Short Tail [Member]
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2]	3
PPD, Gross		(185)
prior period development, net adjustments		1
Prior Year Claims and Claims Adjustment Expense		(184)	(85)	(102)
North America Commercial P&C Insurance | Short Tail [Member] | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(188)
North America Commercial P&C Insurance | Short Tail [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		0
North America Commercial P&C Insurance - Other Casualty [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		0
North America Commercial P&C Insurance - Other Casualty [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		14
North America Commercial P&C Insurance - Non-Casualty [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(188)
North America Commercial P&C Insurance - Non-Casualty [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		0
North America Personal P&C Insurance [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		76
Prior Year Claims and Claims Adjustment Expense		69	27	25
North America Personal P&C Insurance [Member] | Alternative Risk Solutions [Member]
supplementary PPD reconciliation [Line Items]
Prior Year Claims and Claims Adjustment Expense		(55)
North America Personal P&C Insurance [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(10)
North America Personal P&C Insurance [Member] | Long Tail [Member]
supplementary PPD reconciliation [Line Items]
Prior Year Claims and Claims Adjustment Expense		0	0	0
North America Personal P&C Insurance [Member] | Short Tail [Member]
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2]	(7)
PPD, Gross		69
prior period development, net adjustments		0
Prior Year Claims and Claims Adjustment Expense		69	27	25
North America Personal P&C Insurance [Member] | Short Tail [Member] | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		86
North America Personal P&C Insurance [Member] | Short Tail [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(10)
Overseas General Insurance
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2],[4]	(43)
PPD, Gross		(252)
prior period development, net adjustments		0
Prior Year Claims and Claims Adjustment Expense		(252)	(423)	(343)
Overseas General Insurance | International A&H [Member]
supplementary PPD reconciliation [Line Items]
Prior Year Claims and Claims Adjustment Expense		(35)
Overseas General Insurance | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(193)
Overseas General Insurance | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(16)
Overseas General Insurance | Long Tail [Member]
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2]	(3)
PPD, Gross		(71)
prior period development, net adjustments		0
Prior Year Claims and Claims Adjustment Expense		(71)	(236)	(192)
Overseas General Insurance | Long Tail [Member] | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(55)
Overseas General Insurance | Long Tail [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(13)
Overseas General Insurance | Short Tail [Member]
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2]	(40)
PPD, Gross		(181)
prior period development, net adjustments		0
Prior Year Claims and Claims Adjustment Expense		(181)	(187)	(151)
Overseas General Insurance | Short Tail [Member] | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(138)
Overseas General Insurance | Short Tail [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(3)
Overseas General Insurance - Casualty [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(68)
Overseas General Insurance - Casualty [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(13)
Overseas General Insurance - Non-Casualty [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(141)
Overseas General Insurance - Non-Casualty [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(3)
Segment Global Reinsurance [Member]
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2]	1
PPD, Gross		(55)
prior period development, net adjustments		(4)
Prior Year Claims and Claims Adjustment Expense		(59)
Segment Global Reinsurance [Member] | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		4
Segment Global Reinsurance [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(60)
Segment Global Reinsurance [Member] | Long Tail [Member]
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2]	1
PPD, Gross		(71)
prior period development, net adjustments		3
Prior Year Claims and Claims Adjustment Expense		(68)
Segment Global Reinsurance [Member] | Long Tail [Member] | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(12)
Segment Global Reinsurance [Member] | Long Tail [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(60)
Segment Global Reinsurance [Member] | Short Tail [Member]
supplementary PPD reconciliation [Line Items]
Other PPD adjustments	[2]	0
PPD, Gross		16
prior period development, net adjustments		(7)
Prior Year Claims and Claims Adjustment Expense		9
Segment Global Reinsurance [Member] | Short Tail [Member] | 2008 to 2016 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		16
Segment Global Reinsurance [Member] | Short Tail [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		0
Global Reinsurance - Casualty [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(72)
Global Reinsurance - Casualty [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		(60)
Global Reinsurance - Non-Casualty [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		16
Global Reinsurance - Non-Casualty [Member] | prior to 2008 [Member]
supplementary PPD reconciliation [Line Items]
short-duration PPD		0
North America Agricultural Insurance
supplementary PPD reconciliation [Line Items]
Prior Year Claims and Claims Adjustment Expense		(119)	(72)	(45)
North America Agricultural Insurance | Long Tail [Member]
supplementary PPD reconciliation [Line Items]
Prior Year Claims and Claims Adjustment Expense		0	0	0
North America Agricultural Insurance | Short Tail [Member]
supplementary PPD reconciliation [Line Items]
PPD, Gross		(174)
prior period development, net adjustments		55
Prior Year Claims and Claims Adjustment Expense		(119)	(72)	(45)
Corporate Segment [Member]
supplementary PPD reconciliation [Line Items]
Prior Year Claims and Claims Adjustment Expense		278	189	200
Corporate Segment [Member] | Long Tail [Member]
supplementary PPD reconciliation [Line Items]
PPD, Gross		278
prior period development, net adjustments		0
Prior Year Claims and Claims Adjustment Expense		278	189	200
Corporate Segment [Member] | Short Tail [Member]
supplementary PPD reconciliation [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 0	$ 0	$ 0

[1] (2) Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments, and earned premiums totaling $108 million, $69 million, and nil, for 2017, 2016, and 2015, respectively.

[2] (1) Other includes the impact of foreign exchange.

[3] (2) Includes favorable development of $55 million related to our Alternative Risk Solutions business; the remaining difference relates to a number of other items, none of which are individually material.

[4] (3) Includes favorable development of $35 million related to International A&H business, the remaining difference relates to a number of other items, none of which are individually material.