Reinsurance - Claims expense, life contract benefits and interest credited to contractholder funds (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 claims and claims expense
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	$ (138)	$ (124)	$ (325)	$ (255)
Life contract benefits
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	(66)	(70)	(115)	(117)
Interest credited to contractholder funds
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	$ (7)	$ (6)	$ (11)	$ (11)