Reinsurance - Premiums earned and life premiums and contract charges (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Property and casualty insurance premiums
Reinsurance
Property and casualty insurance premiums earned	$ (257)	$ (254)	$ (496)	$ (500)
Life premiums
Reinsurance
Life premiums and contract charges	$ (73)	$ (75)	$ (145)	$ (150)