Unpaid losses and loss expenses Unpaid losses and loss expenses (narrative details) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	[1]			$ (781)	$ (972)
prior period development, net adjustments				110	75
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		$ (236)	$ (187)	(546)	(533)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(242)	(167)	(431)	(473)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		6		(115)	(60)
North America Commercial P&C Insurance [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(99)	(134)
North America Commercial P&C Insurance [Member] | Multi-Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
prior period development, net adjustments		26
North America Commercial P&C Insurance [Member] | Accident Year 2016 [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(37)
North America Commercial P&C Insurance [Member] | Accident Year 2013 and prior [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(44)		(62)
North America Commercial P&C Insurance [Member] | Accident years 2008 - 2013 [Member] | Commercial Multi-peril and General Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(27)
North America Commercial P&C Insurance [Member] | Accident years 2014 and 2016 [Member] | Property [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(43)
North America Commercial P&C Insurance [Member] | Accident Year 2010 and Prior [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(127)		(272)
North America Commercial P&C Insurance [Member] | Accident year 2012 and 2013 [Member] | Professional Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(30)
North America Commercial P&C Insurance [Member] | Accident year 2012 and 2013 [Member] | Multi-Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(28)		(28)
North America Commercial P&C Insurance [Member] | Accident year 2015 and 2016 [Member] | Accident and Health Insurance Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(19)
North America Commercial P&C Insurance [Member] | Accident year 2011 and prior [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(140)		(213)
North America Commercial P&C Insurance [Member] | Accident Year 2015 [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse					(40)
North America Commercial P&C Insurance [Member] | Accident Year 2015 [Member] | Credit Related Business [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(45)
North America Commercial P&C Insurance [Member] | Accident year 2007 - 2014 [Member] | Primary Casualty GL [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse					(20)
North America Commercial P&C Insurance [Member] | Accident years 2012 and prior [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse					(91)
North America Commercial P&C Insurance [Member] | Accident years 2012 and prior [Member] | Professional Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse					(69)
North America Commercial P&C Insurance [Member] | Accident year 2014 and 2015 [Member] | Property [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(20)		(46)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		32	38	66	20
North America Personal P&C Insurance [Member] | Personal Homeowner [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			28	105
North America Personal P&C Insurance [Member] | Accident Year 2016 [Member] | Personal Homeowner [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		98
North America Personal P&C Insurance [Member] | Accident Year 2014 [Member] | Personal Excess [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(58)		(58)
North America Agricultural Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(4)	(11)
North America Agricultural Insurance [Member] | Commercial Multi-peril and General Liability [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(83)	(52)
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(108)	(223)	(184)	(338)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(109)	(234)	(78)	(235)
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			11	(106)	(103)
Overseas General Insurance [Member] | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(40)		(9)
Overseas General Insurance [Member] | Casualty and Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(167)		(167)
Overseas General Insurance [Member] | Individual Legacy Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(25)
Overseas General Insurance [Member] | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(34)		(34)
Overseas General Insurance [Member] | Accident Year 2013 and prior [Member] | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(69)
Overseas General Insurance [Member] | Accident Year 2013 and prior [Member] | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(124)
Overseas General Insurance [Member] | Accident year 2013 to 2015 [Member] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(37)
Overseas General Insurance [Member] | Accident year 2013 to 2015 [Member] | Casualty and Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			94		94
Overseas General Insurance [Member] | Accident years 2014 - 2016 [Member] | Accident and Health Insurance Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(20)
Overseas General Insurance [Member] | Accident years 2014 - 2016 [Member] | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		29
Overseas General Insurance [Member] | Accident years 2014 - 2016 [Member] | Energy and Technical Risk Property [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(43)
Overseas General Insurance [Member] | Accident years 2014 - 2016 [Member] | Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		90
Overseas General Insurance [Member] | Accident years 2012 and prior [Member] | Casualty and Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(261)		(261)
Overseas General Insurance [Member] | Accident years 2012 and prior [Member] | Aviation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(28)
Overseas General Insurance [Member] | Accident years 2016 and prior [Member] | Casualty and Financial [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				32
Overseas General Insurance [Member] | Accident year 2012 to 2014 [Member] | Property [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse					(66)
Overseas General Insurance [Member] | Accident year 2010 to 2014 [Member] | Energy and Technical Risk Property [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse					(35)
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(41)	(28)	(64)	(78)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(67)
Global Reinsurance [Member] | Auto Liability Excess Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				10
Global Reinsurance [Member] | Accident Year 2013 and prior [Member] | Professional Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(29)		(66)
Global Reinsurance [Member] | Accident year 2011 and prior [Member] | Professional Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(24)		(30)
Global Reinsurance [Member] | Accident year 2011 and prior [Member] | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse					(41)
Global Reinsurance [Member] | Accident Year 2015 [Member] | Auto Liability Excess Lines [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				9
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		87	62	140	84
Corporate Segment [Member] | Discontinued Operations [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		9	10	28	$ 27
Corporate Segment [Member] | non-A&E run-off [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				$ 35
Corporate Segment [Member] | Asbestos & Environmental [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		$ 77	$ 52

[1] Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments, and earned premiums totaling $110 million and $75 million for the nine months ended September 30, 2017 and 2016, respectively.