Investment Securities (Insurance-Linked Securities Funds) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,541.6	$ 2,746.0	$ 2,534.6	$ 2,328.7
Minimum
Debt and Equity Securities, FV-NI [Line Items]
Redemptions and advanced redemption notice period	30 days
Maximum
Debt and Equity Securities, FV-NI [Line Items]
Redemptions and advanced redemption notice period	90 days
Insurance-Linked Securities Funds
Debt and Equity Securities, FV-NI [Line Items]
Number of Investments	4
Fair value investments	$ 46.2
Unfunded Capital Commitment	$ 5.2