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Business Combinations (Details) - USD ($)
$ in Millions
9 Months Ended
Apr. 06, 2018
Sep. 30, 2018
Dec. 31, 2017
Business Acquisition [Line Items]      
Proceeds from line of credit   $ 737.2  
Goodwill   $ 7,311.5 $ 6,590.7
Guggenheim Investments' ETF Business [Member]      
Business Acquisition [Line Items]      
Purchase price $ 1,200.0    
Goodwill 676.9    
Line of Credit | Guggenheim Investments' ETF Business [Member]      
Business Acquisition [Line Items]      
Proceeds from line of credit 835.0    
Fund Management Contracts | Guggenheim Investments' ETF Business [Member]      
Business Acquisition [Line Items]      
Intangible assets acquired $ 527.0