Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Sales and maturities of investment securities	$ 2,164.4	$ 2,791.3	$ 3,692.4
Common Stock
Investment securities
Cost or amortized cost	904.7	739.7
Gross unrealized gains	51.0	129.4
Gross unrealized losses	(30.1)	(3.0)
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	925.6	866.1
Other long-term investments
Investment securities
Cost or amortized cost	330.3	246.6
Gross unrealized gains	52.2	6.8
Gross unrealized losses	(54.9)	(39.7)
Net foreign currency gains	(2.0)	(4.9)
Fixed maturity investments, at fair value	$ 325.6	$ 208.8