Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fixed maturity investments, at fair value	$ 2,639.7	$ 2,422.0
Sales and maturities of investment securities	2,162.8	3,337.5	$ 2,672.8
Common Stock
Investment securities
Trading Securities, Cost	822.5	495.3
Net foreign currency gains (losses)	0.0	(7.4)
Fixed maturity investments, at fair value	1,113.9	611.7
Trading Securities, Unrealized Holding Gain	302.8	127.4
Trading Securities, Unrealized Holding Loss	(11.4)	(3.6)
Other long-term investments
Investment securities
Trading Securities, Cost	304.5	286.7
Net foreign currency gains (losses)	(2.3)	(1.3)
Fixed maturity investments, at fair value	315.8	331.9
Trading Securities, Unrealized Holding Gain	32.0	53.3
Trading Securities, Unrealized Holding Loss	$ (18.4)	$ (6.8)