Catastrophe Reinsurance - Catastrophe Reinsurance Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	$ 31.2	$ 27.4	$ 31.6
Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	26.7	20.5	20.5
Specialty Property & Casualty Insurance | Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	4.8	0.2	2.6
Preferred Property & Casualty Insurance | Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	20.7	20.2	17.8
Life & Health Insurance | Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	$ 1.2	$ 0.1	$ 0.1