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Investments (Narrative) (Details)
3 Months Ended
Mar. 31, 2017
USD ($)
fund
Mar. 31, 2016
USD ($)
Dec. 31, 2016
fund
Schedule of Available-for-sale Securities [Line Items]      
Trading Securities, Change in Unrealized Holding Gain (Loss) $ 4,800,000 $ (1,700,000)  
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments $ 600,000 300,000  
Number of affiliated funds holding seed money | fund 92   103
Seed Money [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Other-than-temporary impairment charges on seed money investments $ 0 $ 0