Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fixed maturity investments, at fair value	$ 4,250.2	$ 2,630.2
Sales and maturities of investment securities	5,385.3	2,162.8	$ 3,337.5
Common Stock
Investment securities
Trading Securities, Cost	440.8	822.5
Net foreign currency gains	0.0	0.0
Fixed maturity investments, at fair value	474.3	1,113.9
Trading Securities, Unrealized Holding Gain	35.9	302.8
Trading Securities, Unrealized Holding Loss	(2.4)	(11.4)
Other long-term investments
Investment securities
Trading Securities, Cost	314.9	304.5
Net foreign currency gains	(3.9)	(2.3)
Fixed maturity investments, at fair value	323.3	315.8
Trading Securities, Unrealized Holding Gain	40.3	32.0
Trading Securities, Unrealized Holding Loss	$ (28.0)	$ (18.4)