Unpaid losses and loss expenses Unpaid losses and loss expenses (narrative details) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			$ (461)	$ (601)
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	$ (131)	$ (168)	(310)	(346)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(101)	(167)	(200)	(309)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(30)		(110)	(37)
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	(83)	(114)	(84)	(114)
North America Commercial P&C Insurance [Member] | Primary casualty GL [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(24)
North America Commercial P&C Insurance [Member] | construction defects [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				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	(57)
North America Commercial P&C Insurance [Member] | Accident Year 2015 and prior [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 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	(37)		(37)
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			(33)
North America Commercial P&C Insurance [Member] | Accident years 2014 - 2016 [Member] | Commercial Property [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(29)
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			(74)	(145)
North America Commercial P&C Insurance [Member] | Accident year 2012 and 2013 [Member] | Professional Malpractice Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(25)
North America Commercial P&C Insurance [Member] | Accident year 2011 and prior [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(59)
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			(24)
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] | Commercial Multi-peril and General Liability [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(50)		(50)
North America Commercial P&C Insurance [Member] | Accident year 2003 [Member] | Professional Malpractice Liability Insurance [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] | Professional Malpractice Liability Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(63)
North America Commercial P&C Insurance [Member] | Accident Year 2013 [Member] | Credit related business [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(24)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	37	(15)	34	(18)
North America Agricultural Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	0	0	(79)	(41)
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(88)	(85)	(76)	(115)
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(88)	(84)	(108)
Overseas General Insurance [Member] | Energy and Technical Risk Property [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(32)
Overseas General Insurance [Member] | Accident year 2013 to 2015 [Member] | Property [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(38)
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 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	(26)
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	(19)
Overseas General Insurance [Member] | Accident years 2016 and prior [Member] | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			32
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(31)	(47)	(23)	(50)
Global Reinsurance [Member] | Casualty & Professional [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(27)
Global Reinsurance [Member] | Accident year 2011 and prior [Member] | Casualty & Professional [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(36)
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	43	$ 14	53	$ 22
Corporate Segment [Member] | Discontinued Operations [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	8		$ 18
Corporate Segment [Member] | non-A&E run-off [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	$ 35