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Investment Securities (Debt securities issued by corporation) (Details) - USD ($)
$ in Millions
Jun. 30, 2016
Dec. 31, 2015
Jun. 30, 2015
Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments $ 5,091.8 $ 4,050.9 $ 3,379.6 $ 3,343.6
Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 1,066.1 1,000.0    
AAA | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 0.0 0.0    
AA | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 125.7 95.2    
A | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 376.6 397.7    
BBB | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments 563.8 507.1    
Other | Debt securities issued by corporations        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Fair value investments $ 0.0 $ 0.0