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Ceded Reinsurance (Credit Risk by Reinsurer) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Ceded Credit Risk [Line Items]        
Reinsurance recoverable $ 1,625.0 $ 1,480.0 $ 1,340.3 $ 1,234.5
Prepaid reinsurance 486.4 392.3    
Reinsurance recoverable        
Ceded Credit Risk [Line Items]        
Reinsurance recoverable $ 1,625.0 $ 1,480.0    
Percentage of total 100.00% 100.00%    
Reinsurance recoverable | Top five reinsurers        
Ceded Credit Risk [Line Items]        
Reinsurance recoverable $ 858.7 $ 771.9    
Percentage of total 52.80% 52.20%    
Reinsurance recoverable | Top five reinsurers | A plus | Munich Re        
Ceded Credit Risk [Line Items]        
Reinsurance recoverable $ 348.1 $ 307.9    
Percentage of total 21.40% 20.80%    
Reinsurance recoverable | Top five reinsurers | A plus | Axis Capital        
Ceded Credit Risk [Line Items]        
Reinsurance recoverable $ 160.4 $ 150.8    
Percentage of total 9.90% 10.20%    
Reinsurance recoverable | Top five reinsurers | A plus | Swiss Re        
Ceded Credit Risk [Line Items]        
Reinsurance recoverable $ 146.7 $ 103.9    
Percentage of total 9.00% 7.00%    
Reinsurance recoverable | Top five reinsurers | A plus | Arch Re        
Ceded Credit Risk [Line Items]        
Reinsurance recoverable $ 100.5 $ 115.3    
Percentage of total 6.20% 7.80%    
Reinsurance recoverable | Top five reinsurers | A plus | RenaissanceRe        
Ceded Credit Risk [Line Items]        
Reinsurance recoverable   $ 94.0    
Percentage of total   6.40%    
Reinsurance recoverable | Top five reinsurers | A rating | Markel        
Ceded Credit Risk [Line Items]        
Reinsurance recoverable $ 103.0      
Percentage of total 6.30%      
Reinsurance recoverable | Other reinsurers' balances        
Ceded Credit Risk [Line Items]        
Reinsurance recoverable $ 766.3 $ 708.1    
Percentage of total 47.20% 47.80%    
Prepaid Reinsurance [Member]        
Ceded Credit Risk [Line Items]        
Prepaid reinsurance $ 486.4 $ 392.3    
Percentage of total 100.00% 100.00%    
Prepaid Reinsurance [Member] | Top five reinsurers        
Ceded Credit Risk [Line Items]        
Prepaid reinsurance [1] $ 250.1 $ 175.2    
Percentage of total 51.40% 44.60%    
Prepaid Reinsurance [Member] | Top five reinsurers | A plus | Munich Re        
Ceded Credit Risk [Line Items]        
Prepaid reinsurance [1] $ 87.5 $ 61.7    
Percentage of total 18.00% 15.70%    
Prepaid Reinsurance [Member] | Top five reinsurers | A plus | Axis Capital        
Ceded Credit Risk [Line Items]        
Prepaid reinsurance [1] $ 58.5 $ 32.2    
Percentage of total 12.00% 8.20%    
Prepaid Reinsurance [Member] | Top five reinsurers | A plus | Swiss Re        
Ceded Credit Risk [Line Items]        
Prepaid reinsurance [1] $ 64.2 $ 57.3    
Percentage of total 13.20% 14.60%    
Prepaid Reinsurance [Member] | Top five reinsurers | A plus | Arch Re        
Ceded Credit Risk [Line Items]        
Prepaid reinsurance [1] $ 14.0 $ 16.0    
Percentage of total 2.90% 4.10%    
Prepaid Reinsurance [Member] | Top five reinsurers | A plus | RenaissanceRe        
Ceded Credit Risk [Line Items]        
Prepaid reinsurance [1]   $ 8.0    
Percentage of total   2.00%    
Prepaid Reinsurance [Member] | Top five reinsurers | A rating | Markel        
Ceded Credit Risk [Line Items]        
Prepaid reinsurance [1] $ 25.9      
Percentage of total 5.30%      
Prepaid Reinsurance [Member] | Other reinsurers' balances        
Ceded Credit Risk [Line Items]        
Prepaid reinsurance [1] $ 236.3 $ 217.1    
Percentage of total 48.60% 55.40%    
[1] Prepaid reinsurance represents unearned premiums ceded to reinsurance companies.