Unpaid losses and loss expenses Unpaid losses and loss expenses (Details) (Supplementary PPD) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
North America Commercial P&C Insurance - Workers' Compensation [Member]
Supplementary PPD [Line Items]
short-duration PPD	$ (108)
North America Commercial P&C Insurance - Workers' Compensation [Member] | prior to 2008 [Member]
Supplementary PPD [Line Items]
short-duration PPD	(35)
North America Commercial P&C Insurance - Liability [Member]
Supplementary PPD [Line Items]
short-duration PPD	(434)
North America Commercial P&C Insurance - Liability [Member] | prior to 2008 [Member]
Supplementary PPD [Line Items]
short-duration PPD	(154)
North America Commercial P&C Insurance - Other Casualty [Member]
Supplementary PPD [Line Items]
short-duration PPD	0
North America Commercial P&C Insurance - Other Casualty [Member] | prior to 2008 [Member]
Supplementary PPD [Line Items]
short-duration PPD	14
North America Commercial P&C Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
short-duration PPD	(188)
North America Commercial P&C Insurance - Non-Casualty [Member] | prior to 2008 [Member]
Supplementary PPD [Line Items]
short-duration PPD	0
North America Personal P&C Insurance [Member]
Supplementary PPD [Line Items]
short-duration PPD	76
North America Personal P&C Insurance [Member] | prior to 2008 [Member]
Supplementary PPD [Line Items]
short-duration PPD	(10)
Overseas General Insurance - Casualty [Member]
Supplementary PPD [Line Items]
short-duration PPD	(68)
Overseas General Insurance - Casualty [Member] | prior to 2008 [Member]
Supplementary PPD [Line Items]
short-duration PPD	(13)
Overseas General Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
short-duration PPD	(141)
Overseas General Insurance - Non-Casualty [Member] | prior to 2008 [Member]
Supplementary PPD [Line Items]
short-duration PPD	(3)
Global Reinsurance - Casualty [Member]
Supplementary PPD [Line Items]
short-duration PPD	(72)
Global Reinsurance - Casualty [Member] | prior to 2008 [Member]
Supplementary PPD [Line Items]
short-duration PPD	(60)
Global Reinsurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
short-duration PPD	16
Global Reinsurance - Non-Casualty [Member] | prior to 2008 [Member]
Supplementary PPD [Line Items]
short-duration PPD	$ 0