Investment Securities (Insurance-Linked Securities Funds) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,656.9	$ 2,746.0	$ 2,483.0	$ 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	$ 52.1