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Loss And Loss Adjustment Expense Reserves Reconciliation of the Claims Development to the Liability for Claims and Claim Adjustment Expenses (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance $ 10,029.8 $ 8,800.0    
Total reinsurance recoverable on unpaid claims 2,157.8 1,795.9    
Unallocated claims adjustment expense related to: 887.0 767.0    
Total gross liability for unpaid claims and claim adjustment expense 13,086.9 11,368.0 $ 10,039.0 $ 8,857.4
Other business        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 43.7 37.1    
Total reinsurance recoverable on unpaid claims 200.9 154.9    
Commercial Lines Segment | Commercial Lines - Liability        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 2,027.4 1,675.3    
Total reinsurance recoverable on unpaid claims 105.1 59.5    
Commercial Lines Segment | Commercial Lines - Physical Damage        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 51.7 46.6    
Total reinsurance recoverable on unpaid claims 0.1 0.0    
Property Segment | Property        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 245.8 224.7    
Total reinsurance recoverable on unpaid claims 243.8 132.7    
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item 12.3 5.1    
Agency Channel | Personal Lines Segment | Personal Lines - Liability        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 3,955.5 3,555.9    
Total reinsurance recoverable on unpaid claims 769.8 724.5    
Agency Channel | Personal Lines Segment | Personal Lines - Physical Damage        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 26.2 30.1    
Total reinsurance recoverable on unpaid claims 0.0 0.0    
Direct Channel | Personal Lines Segment | Personal Lines - Liability        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 3,685.1 3,219.1    
Total reinsurance recoverable on unpaid claims 838.1 724.3    
Direct Channel | Personal Lines Segment | Personal Lines - Physical Damage        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance (5.6) 11.2    
Total reinsurance recoverable on unpaid claims $ 0.0 $ 0.0