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Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Reconciliation of Claims Development to Liability) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Net outstanding liabilities $ 1,345,074 $ 1,234,797 $ 1,148,541  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 3,041,694      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 585,855 611,200 551,019 $ 571,978
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense 143,691      
Liability for Claims and Claims Adjustment Expense 3,771,240 $ 3,691,719 $ 3,517,728 $ 3,477,870
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 1,092,058      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 175,276      
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 279,023      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 21,360      
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 916,471      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 218,024      
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 464,131      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 16,745      
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 83,163      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 3,926      
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 50,300      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 24,387      
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 13,519      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 389      
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 10,560      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 2,287      
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 2,616,683      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 440,645      
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 92,557      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 53,129      
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 30,728      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 999      
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 9,184      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 69,333      
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 132,469      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 123,461      
Total E&S 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 292,542      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments $ 21,749