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Investments (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Investment [Line Items]    
Available-for-sale securities $ 18,951.0 $ 19,909.9
Carrying value of fixed maturity investments that did not produce income 0.2 9.2
Future capital calls from various third-party investments 662.3  
Securities on deposit under regulatory requirements $ 558.2 $ 504.4
Securities lending transactions, ratio of fair value of collateral on hold to fair value of securities on loan 103.00% 103.00%
Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Available-for-sale securities $ 2,155.2  
Asset-Backed Securities [Member]    
Investment [Line Items]    
Available-for-sale securities 698.6  
Sub-Prime Mortgage-Backed And Asset-Backed Securities [Member]    
Investment [Line Items]    
Available-for-sale securities 30.9  
Accumulated net unrealized gains (losses) 1.0  
Alt-A Mortgage Backed And Asset Backed Securities [Member]    
Investment [Line Items]    
Available-for-sale securities 58.2  
Accumulated net unrealized gains (losses) 3.3  
Energy Sector Fixed Maturity Securities [Member]    
Investment [Line Items]    
Available-for-sale securities 777.2  
Accumulated net unrealized gains (losses) $ (172.0)