Unpaid losses and loss expenses Unpaid losses and loss expenses (narrative)(Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	$ (191)	$ (170)	$ (400)	$ (401)
Prior period reclassification adjustments		(1)		(13)
non-A&E run-off [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	45		55
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(91)	(76)	(73)	(120)
Prior period reclassification adjustments		(2)		3
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(100)	(94)	(327)	(281)
Accident year 2017 [Member] | Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(36)		(142)
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(155)	(131)	(256)	(310)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(104)	(83)	(96)	(177)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(51)	(48)	(160)	(133)
North America Commercial P&C Insurance [Member] | Surety [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(19)
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	(118)	(75)	(138)	(79)
North America Commercial P&C Insurance [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(29)
North America Commercial P&C Insurance [Member] | Other [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 2017 [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(56)
North America Commercial P&C Insurance [Member] | Accident year 2017 [Member] | Catastrophe [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(3)
North America Commercial P&C Insurance [Member] | Accident year 2017 [Member] | Catastrophe [Member] | Commercial Property and Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(40)		(115)
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		(38)
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)
North America Commercial P&C Insurance [Member] | Accident years 2015 through 2017 [Member] | Other [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	14		74
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 2012 and 2013 [Member] | Professional 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 years 2014 and 2016 [Member] | Property [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(45)		(57)
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 2015 [Member] | Surety [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(45)
North America Personal [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	7		1
North America Personal [Member] | Accident year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(5)		(11)
North America Personal [Member] | Accident Years 2012 - 2016 [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		37		34
North America Agricultural Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	0	0	(76)	(79)
North America Agricultural Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	0	0	0	0
North America Agricultural Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	0	0	(76)	(79)
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(72)	(88)	(94)	(76)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(2)	0	(2)	32
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(70)	(88)	(92)	(108)
Overseas General Insurance [Member] | Personal lines [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(17)
Overseas General Insurance [Member] | Accident and Health Insurance Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(16)
Overseas General Insurance [Member] | Other [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(30)
Overseas General Insurance [Member] | Accident year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(12)
Overseas General Insurance [Member] | Accident year 2013 - 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] | Property and Inland Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(40)
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	(16)	(31)	(30)	(23)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(30)
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	14
Global Reinsurance [Member] | Other [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(27)
Global Reinsurance [Member] | Accident Year 2013 and prior [Member] | Casualty & Professional [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(32)
Global Reinsurance [Member] | Accident year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	9		(1)
Global Reinsurance [Member] | Accident year 2011 and prior [Member] | Casualty & Professional [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(36)		(36)
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
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(16)	(31)	(30)	(23)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(30)	(36)	(30)	(28)
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	14	5	0	5
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	45	43	55	53
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] | Discontinued Operations [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	10	8	20	18
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	45	43	55	53
Corporate Segment [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	0	$ 0	0	$ 0
All segments excluding Corporate [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(236)		(455)
All segments excluding Corporate [Member] | Excluding Catastrophic Events [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(64)		(188)
All segments excluding Corporate [Member] | Accident years 2014 and prior [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	$ (136)		$ (125)