Reinsurance and indemnification - Claims expense, life contract benefits and interest credited to contractholder funds (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Reductions to costs and expenses due to reinsurance ceded amounts
Unearned premiums	$ 20,248		$ 19,844
Nationwide Reinsurance Program
Reductions to costs and expenses due to reinsurance ceded amounts
Unearned premiums		$ 75
Property and casualty insurance claims and claims expense
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	(109)	(1,593)
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	(12)	(386)
Property and casualty insurance claims and claims expense | Nationwide Reinsurance Program
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred		(955)
Accident, health and other policy benefits
Reductions to costs and expenses due to reinsurance ceded amounts
Ceded losses incurred	$ (7)	$ (29)