Investment Securities (Investment Gains (Losses) For Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ (267.3)	$ 152.9	$ (79.3)
Realized gains	315.5	112.9	194.0
Realized losses	35.2	35.6	27.2
Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	6.2	(23.1)	7.7
Fixed maturity investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(12.1)	(15.6)	11.3
Fixed maturity investments | Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.1	(1.1)	1.9
Common equity securities
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(254.6)	207.6	(83.0)
Common equity securities | Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.0	(9.0)	5.8
Other long-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(0.6)	(39.1)	(7.6)
Other long-term investments | Level 3 Inputs
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 6.1	$ (13.0)	$ 0.0