Reinsurance - Pennsylvania, Kentucky, Florida and Southern States Reinsurance Agreement (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) contract limit
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	$ 500
Pennsylvania Agreement
Ceded Credit Risk [Line Items]
Term of catastrophe reinsurance agreement contract	3 years
Number of limits | limit	3
Limit of aggregate losses under aggregate excess agreement	$ 100
Retention amount for catastrophe reinsurance agreement contracts	$ 100
Number of limits available per contract year | limit	2
Contracts number | contract	1
Retention percentage of reinsurance limit	95.00%
Kentucky Agreement
Ceded Credit Risk [Line Items]
Term of catastrophe reinsurance agreement contract	3 years
Number of limits | limit	3
Limit of aggregate losses under aggregate excess agreement	$ 28
Retention amount for catastrophe reinsurance agreement contracts	$ 2
Number of limits available per contract year | limit	2
Contracts number | contract	1
Retention percentage of reinsurance limit	95.00%
Aggregate Excess Catastrophe Florida Reinsurance Contract
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 200
Southeast States Reinsurance Contract
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	300
Retention amount for catastrophe reinsurance agreement contracts	$ 200