Unpaid losses and loss expenses Unpaid losses and loss expenses (Details) (Supplementary PPD) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
North America Commercial P&C Insurance - Workers' Compensation [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	$ (155)
North America Commercial P&C Insurance - Workers' Compensation [Member] | prior to 2009 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(73)
North America Commercial P&C Insurance - Liability [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(141)
North America Commercial P&C Insurance - Liability [Member] | prior to 2009 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(3)
North America Commercial P&C Insurance - Other Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	20
North America Commercial P&C Insurance - Other Casualty [Member] | prior to 2009 [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	(224)
North America Commercial P&C Insurance - Non-Casualty [Member] | prior to 2009 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(1)
North America Personal P&C Insurance [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	47
North America Personal P&C Insurance [Member] | prior to 2009 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(7)
Overseas General Insurance - Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(64)
Overseas General Insurance - Casualty [Member] | prior to 2009 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(55)
Overseas General Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(109)
Overseas General Insurance - Non-Casualty [Member] | prior to 2009 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(11)
Global Reinsurance - Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(73)
Global Reinsurance - Casualty [Member] | prior to 2009 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(48)
Global Reinsurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	18
Global Reinsurance - Non-Casualty [Member] | prior to 2009 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	$ (2)