Investments (Gross Unrealized Holding Gains And Losses Recognized In Other Accumulated Comprehensive Income From Available-For-Sale Investments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 65.7	$ 89.9
Gross Unrealized Holding Gains	2.2	10.8
Gross Unrealized Holding Losses	(4.4)	(0.7)
Fair Value	63.5	100.0
Seed Money [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	65.7	89.6
Gross Unrealized Holding Gains	2.2	10.6
Gross Unrealized Holding Losses	(4.4)	(0.7)
Fair Value	63.5	99.5
Current Available-For-Sale Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	65.7	89.6
Gross Unrealized Holding Gains	2.2	10.6
Gross Unrealized Holding Losses	(4.4)	(0.7)
Fair Value	63.5	99.5
CLO [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	0	0.3
Gross Unrealized Holding Gains	0	0.2
Gross Unrealized Holding Losses	0	0
Fair Value	0	0.5
Non-current available-for-sale investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	0	0.3
Gross Unrealized Holding Gains	0	0.2
Gross Unrealized Holding Losses	0	0
Fair Value	$ 0	$ 0.5