Investment Securities (Insurance-Linked Securities Funds) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Investment [Line Items]
Fair value investments	$ 2,569.4	$ 2,328.7	$ 2,365.1	$ 3,208.3
Insurance-Linked Securities Funds
Investment [Line Items]
Number of Investments	3
Fair value investments	$ 41.4
Minimum
Investment [Line Items]
Redemptions and advanced redemption notice period	30 days
Maximum
Investment [Line Items]
Redemptions and advanced redemption notice period	90 days