Catastrophe Reinsurance - Catastrophe Reinsurance Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Catastrophe Claims [Line Items]
Ceded Catastrophe Reinsurance Premiums	$ 31.6	$ 18.2	$ 19.0
Catastrophe Reinsurance [Member]
Liability for Catastrophe Claims [Line Items]
Ceded Catastrophe Reinsurance Premiums	20.5	11.0	12.0
Property and Casualty Insurance [Member] | Catastrophe Reinsurance [Member]
Liability for Catastrophe Claims [Line Items]
Ceded Catastrophe Reinsurance Premiums	17.8	10.8	11.8
Specialty Property & Casualty Insurance [Member] | Catastrophe Reinsurance [Member]
Liability for Catastrophe Claims [Line Items]
Ceded Catastrophe Reinsurance Premiums	2.6	0.1	0.1
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	$ 0.4	$ 0.8