Investment Securities - Insurance-Linked Securities Funds (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) fund	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 2,533.4	$ 2,746.0	$ 2,569.4	$ 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 | fund	4
Fair value investments	$ 52.7