Investment Securities Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 152.9	$ (79.3)	$ 40.3
Foreign Currency Transaction Gain (Loss), Unrealized	(4.8)	(9.0)	(1.9)
Net unrealized investment gains (losses), pre-tax	148.1	(88.3)	38.4
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(12.3)	9.9	3.3
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	(0.1)	0.5	0.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(12.4)	10.4	3.3
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	140.6	(69.4)	43.6
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(4.9)	(8.5)	(1.9)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	135.7	(77.9)	41.7
Fixed maturity investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(15.6)	11.3	(49.9)
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0	0.0
Net unrealized investment gains (losses), pre-tax	(15.6)	11.3	(49.9)
Common equity securities
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	207.6	(83.0)	86.1
Foreign Currency Transaction Gain (Loss), Unrealized	(3.7)	(7.7)	(1.9)
Net unrealized investment gains (losses), pre-tax	203.9	(90.7)	84.2
Other long-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(39.1)	(7.6)	4.1
Foreign Currency Transaction Gain (Loss), Unrealized	(1.1)	(1.3)	0.0
Net unrealized investment gains (losses), pre-tax	$ (40.2)	$ (8.9)	$ 4.1