Reinsurance and indemnification - Claims expense, life contract benefits and interest credited to contractholder funds (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Reductions to costs and expenses due to reinsurance ceded amounts
Unearned premiums	$ 18,177		$ 15,946
Property and casualty insurance claims and claims expense
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	(1,593)	$ (131)
Accident and health insurance policy benefits
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	(29)	(6)
Michigan Catastrophic Claim Association | Property and casualty insurance claims and claims expense
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	386	$ 47
Nationwide Reinsurance Program
Reductions to costs and expenses due to reinsurance ceded amounts
Unearned premiums	75
Nationwide Reinsurance Program | Property and casualty insurance claims and claims expense
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	$ 955