Reinsurance - Catastrophe reinsurance (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) contract
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	$ 500
Nationwide Per Occurrence Excess Catastrophe Reinsurance Program
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	4,420
Retention amount for catastrophe reinsurance agreement contracts	$ 500
Catastrophe reinsurance, number of agreements | contract	3