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Fair Value Measurements (Financial Instruments Measured At Fair Value On A Recurring Basis) (Details) - USD ($)
$ in Millions
Mar. 31, 2016
Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Good Faith and Margin Deposits with Broker-Dealers $ 37,497.3 $ 35,553.0
Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Marketable Securities 79.2  
Good Faith and Margin Deposits with Broker-Dealers 10,248.7  
Investment Owned, at Fair Value 322.2  
Total Assets at Fair Value 10,650.1  
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 21.0  
Fair Value, Measurements, Recurring [Member] | Asset Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 0.3  
Fair Value, Measurements, Recurring [Member] | Exchange Traded Funds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Trading Securities at Fair Value 57.8  
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 0.1  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Marketable Securities 78.9  
Good Faith and Margin Deposits with Broker-Dealers 10,248.7  
Investment Owned, at Fair Value 322.2  
Total Assets at Fair Value 10,649.8  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 21.0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Asset Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 0.0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Exchange Traded Funds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Trading Securities at Fair Value 57.8  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 0.1  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Marketable Securities 0.3  
Good Faith and Margin Deposits with Broker-Dealers 0.0  
Investment Owned, at Fair Value 0.0  
Total Assets at Fair Value 0.3  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 0.0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Asset Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 0.3  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Exchange Traded Funds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Trading Securities at Fair Value 0.0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 0.0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Marketable Securities 0.0  
Good Faith and Margin Deposits with Broker-Dealers 0.0  
Investment Owned, at Fair Value 0.0  
Total Assets at Fair Value 0.0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 0.0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Asset Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value 0.0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Exchange Traded Funds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Trading Securities at Fair Value 0.0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale Securities at Fair Value $ 0.0