Investment Securities (Insurance-Linked Securities Funds) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Fair value investments	$ 2,483.0	$ 2,328.7	$ 2,275.4	$ 3,208.3
Insurance-Linked Securities Funds
Investment [Line Items]
Fair value investments	$ 40.3
Minimum
Investment [Line Items]
Redemptions and advanced redemption notice period	30 days
Maximum
Investment [Line Items]
Redemptions and advanced redemption notice period	90 days