Catastrophe Reinsurance - Catastrophe Reinsurance Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Catastrophe Claims [Line Items]
Ceded Catastrophe Reinsurance Premiums	$ 18.2	$ 19.0	$ 20.6
Catastrophe Reinsurance [Member]
Liability for Catastrophe Claims [Line Items]
Ceded Catastrophe Reinsurance Premiums	11.0	12.0	12.9
Property and Casualty Insurance [Member] | Catastrophe Reinsurance [Member]
Liability for Catastrophe Claims [Line Items]
Ceded Catastrophe Reinsurance Premiums	10.9	11.9	12.8
Life and Health Insurance [Member] | Catastrophe Reinsurance [Member]
Liability for Catastrophe Claims [Line Items]
Ceded Catastrophe Reinsurance Premiums	0.1	$ 0.1	$ 0.1
Reinstatement Premium [Member] | Property and Casualty Insurance [Member] | Catastrophe Reinsurance [Member]
Liability for Catastrophe Claims [Line Items]
Ceded Catastrophe Reinsurance Premiums	$ 0.8