Unpaid losses and loss expenses Unpaid losses and loss expenses (narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	$ (243)	$ (270)	$ (643)	$ (671)
prior period development, net adjustments			86	110
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(246)	(290)	(319)	(410)
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	3	20	(324)	(261)
Other [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(377)
Homeowners and Valuables Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	80
Environmental Issue [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	$ 54
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Percentage of PPD	20.00%
Accident years 2014 and prior [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Percentage of PPD	80.00%
North America Commercial P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	$ (216)	(236)	(472)	(546)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(170)	(230)	(266)	(407)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(46)	(6)	(206)	(139)
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			(183)	(125)
North America Commercial P&C Insurance [Member] | Foreign Casualty Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(28)
North America Commercial P&C Insurance [Member] | Multi-Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(28)
prior period development, net adjustments		26
North America Commercial P&C Insurance [Member] | Other [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(50)
North America Commercial P&C Insurance [Member] | Commercial Property and Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(41)		(156)
North America Commercial P&C Insurance [Member] | Surety 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] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Surety Product Line [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 2013 and prior [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(46)		(68)
North America Commercial P&C Insurance [Member] | Accident years 2012 and prior [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(93)		(123)
North America Commercial P&C Insurance [Member] | 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)
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, 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] | Short-duration Insurance Contracts, 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] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(42)
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, 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				(157)
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			71
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				(27)
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)
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, 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 years 2014 and 2016 [Member] | Property Insurance Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(65)
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				(214)
North America Commercial P&C Insurance [Member] | Accident years 2014 and prior [Member] | North America Workers' Compensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(45)		(127)
North America Commercial P&C Insurance [Member] | Accident years 2014 and prior [Member] | Foreign Casualty Line [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(28)
North America Personal [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	58	32	59	66
North America Personal [Member] | Personal Homeowner [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				105
North America Personal [Member] | Homeowners and Valuables Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	80
North America Personal [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Excess [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(24)
North America Personal [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(68)
North America Personal [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Homeowners and Valuables Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(57)
North America Personal [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Excluding Catastrophic Events [Member] | Homeowners and Valuables Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	137
North America Personal [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal Homeowner [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		98
North America Personal [Member] | Short-duration Insurance Contracts, 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]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(1)	(4)	(77)	(83)
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	(1)	(4)	(77)	(83)
North America Agricultural Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(1)		(1)
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(72)	(108)	(166)	(184)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(49)	(106)	(51)	(74)
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(23)	(2)	(115)	(110)
Overseas General Insurance [Member] | Political [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(12)
Overseas General Insurance [Member] | Aviation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(10)
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] | Energy [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(13)
Overseas General Insurance [Member] | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(54)	(40)		(9)
Overseas General Insurance [Member] | Financial | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	38	(34)		(34)
Overseas General Insurance [Member] | Other [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(33)
Overseas General Insurance [Member] | Other [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(57)
Overseas General Insurance [Member] | Surety Product Line [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(17)
Overseas General Insurance [Member] | Environmental Issue [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(10)
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 | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(124)
Overseas General Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	4		(8)
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			(50)
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] | 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] | Financial | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		90
Overseas General Insurance [Member] | Accident years 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 2015 through 2017 [Member] | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	38
Overseas General Insurance [Member] | Accident years 2015 through 2017 [Member] | Financial | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	131
Overseas General Insurance [Member] | Accident Years 2012 - 2015 [Member] | Marine [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(21)
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
Overseas General Insurance [Member] | Accident years 2014 and prior [Member] | Casualty [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(92)
Overseas General Insurance [Member] | Accident years 2014 and prior [Member] | Financial | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(93)
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(24)	(41)	(54)	(64)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(69)
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			15
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] | Casualty & Professional [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse				(66)
Global Reinsurance [Member] | ProfLiabMalpracticeWorkersCompensation [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(39)
Global Reinsurance [Member] | Accident Year 2013 and prior [Member] | Professional Liability Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		(29)
Global Reinsurance [Member] | Accident Year 2013 and prior [Member] | Other [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			(69)
Global Reinsurance [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Catastrophe [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	15		14
Global Reinsurance [Member] | Accident year 2015 and prior [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	(24)	(41)	(54)	(64)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	(39)	(41)	(69)	(69)
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	15	0	15	5
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	12	87	67	140
Corporate Segment [Member] | non-A&E run-off [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse			67	35
Corporate Segment [Member] | Discontinued Operations [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse		9		28
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	12	87	67	140
Corporate Segment [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	0	0	$ 0	$ 0
Corporate Segment [Member] | Environmental Issue [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
PPD (Favorable) / Adverse	$ 54	$ 77