Catastrophe Reinsurance - Catastrophe Losses and LAE (Details) - Catastrophe - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	$ 60.1	$ 87.6	$ 180.4
Specialty Property & Casualty Insurance
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	11.6	4.4	5.2
Preferred Property & Casualty Insurance
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	44.6	79.1	168.8
Life & Health Insurance
Liability for Catastrophe Claims [Line Items]
Catastrophe Losses and LAE	$ 3.9	$ 4.1	$ 6.4