Catastrophe Reinsurance - Catastrophe Losses and LAE (Details) - Catastrophe - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	$ 106.9	$ 60.1	$ 87.6
Specialty Property & Casualty Insurance
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	12.5	11.6	4.4
Preferred Property & Casualty Insurance
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	81.5	44.6	79.1
Life & Health Insurance
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	$ 12.9	$ 3.9	$ 4.1