Investment Securities (Other Long-Term Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
DavidShield & PassportCard
Investment securities
Other Investments	$ 75.0	$ 21.0
Kudu
Investment securities
Other Investments	30.7	0.0
Other unconsolidated private equity investments, at fair value
Investment securities
Other Investments	60.0	62.2
Total unconsolidated entities
Investment securities
Other Investments	165.7	83.2
Hedge Funds and Private Equity Funds
Investment securities
Other Investments	146.1	125.3
Forward Contracts
Investment securities
Other Investments	0.0	(3.7)
Other long-term investments
Investment securities
Other Investments	13.8	4.0
White Mountains
Investment securities
Other Investments	$ 325.6	$ 208.8