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Investments (Gross Unrealized Holding Gains And Losses Recognized In Other Accumulated Comprehensive Income From Available-For-Sale Investments) (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 113.7 $ 149.6
Gross Unrealized Holding Gains 10.3 10.5
Gross Unrealized Holding Losses (2.5) (6.1)
Available for sale investments 121.5 154.0
Seed Money [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 93.3 127.2
Gross Unrealized Holding Gains 7.0 6.8
Gross Unrealized Holding Losses (2.5) (6.1)
Available for sale investments 97.8 127.9
CLO [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 7.5 9.2
Gross Unrealized Holding Gains 3.3 3.7
Gross Unrealized Holding Losses 0.0 0.0
Available for sale investments 10.8 12.9
Other Debt Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Cost 12.9 13.2
Gross Unrealized Holding Gains 0.0 0.0
Gross Unrealized Holding Losses 0.0 0.0
Available for sale investments $ 12.9 $ 13.2