(Net Realized and Unrealized Gains and Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Gain (Loss) on Investments [Abstract]
Net realized investment gains (losses), pre-tax	$ 23.7	$ 38.3	$ 40.5	$ 67.7
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	49.0	(35.4)	82.7	(31.3)
Net Realized and Unrealized Gain (Loss) on Trading Securities	72.7	2.9	123.2	36.4
Trading Securities, Tax on Realized and Unrealized Holding Gain (Loss) on Investments	12.3	1.4	28.3	12.7
Trading Securities Realized and Unrealized Holding Gain (Loss) on Investments after Tax	$ 60.4	$ 1.5	$ 94.9	$ 23.7