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Reserve for Losses and Loss Expenses Reserve for Losses and Loss Expenses (Reconciliation of Claims Development to Liability) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Net outstanding liabilities $ 1,234,797 $ 1,148,541 $ 1,157,501  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 2,944,432      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 611,200 551,019 571,978 $ 540,839
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense 136,087      
Liability for Claims and Claims Adjustment Expense 3,691,719 $ 3,517,728 $ 3,477,870 $ 3,349,770
General Liability [Member] | Standard Commercial Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 1,070,929      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 179,997      
General Liability [Member] | E&S Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 243,613      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 25,741      
Workers Compensation [Member] | Standard Commercial Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 936,766      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 223,327      
Commercial Automobile [Member] | Standard Commercial Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 402,281      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 17,373      
Business Owners' Policies [Member] | Standard Commercial Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 86,802      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 7,012      
Commercial Property [Member] | Standard Commercial Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 42,279      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 13,615      
Commercial Property [Member] | E&S Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 13,251      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 468      
Other Commercial [Member] | Standard Commercial Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 11,389      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 2,613      
Total standard Commercial Lines [Member] | Standard Commercial Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 2,550,446      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 443,937      
Total standard Commercial Lines [Member] | E&S Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 256,864      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 26,209      
Personal Automobile [Member] | Standard Personal Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 90,416      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 55,223      
Homeowners [Member] | Standard Personal Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 38,978      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 3,206      
Other Personal [Member] | Standard Personal Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 7,728      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 82,625      
Standard Personal Lines [Member] | Standard Personal Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 137,122      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments $ 141,054