Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Fixed maturity investments, at fair value		$ 2,081.1
Sales and maturities of investment securities	$ 2,791.3	3,692.4	$ 750.7
Common Stock
Investment securities
Cost or amortized cost	739.7	258.6
Gross unrealized gains	129.4	29.0
Gross unrealized losses	(3.0)	(2.0)
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	866.1	285.6
Other long-term investments
Investment securities
Cost or amortized cost	246.6	194.0
Gross unrealized gains	6.8	7.9
Gross unrealized losses	(39.7)	(25.2)
Net foreign currency gains	(4.9)	(3.9)
Fixed maturity investments, at fair value	$ 208.8	$ 172.8