Reinsurance and indemnification - Claims expense, life contract benefits and interest credited to contractholder funds (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Property and casualty insurance claims and claims expense
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	$ (1,458)	$ 128	$ (3,209)	$ (213)
Property and casualty insurance claims and claims expense | Nationwide Reinsurance Program
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred			(1,400)
Property and casualty insurance claims and claims expense | National General Holdings Corp.
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred			(531)
Property and casualty insurance claims and claims expense | Michigan Catastrophic Claim Association
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred			(514)	(195)
Property and casualty insurance claims and claims expense | WYOF
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred			(185)
Accident and health insurance policy benefits
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	$ (13)	$ (5)	$ (68)	$ (13)