Catastrophe Reinsurance - Catastrophe Reinsurance Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	$ 27.4	$ 31.6	$ 18.2
Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	20.5	20.5	11.0
Specialty Property & Casualty Insurance | Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	0.2	2.6	0.1
Preferred Property & Casualty Insurance | Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	20.2	17.8	10.8
Life & Health Insurance | Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	$ 0.1	$ 0.1	$ 0.1