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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Net outstanding liabilities $ 1,498,134 $ 1,345,074 $ 1,234,797  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 3,216,999      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 537,388 585,855 611,200 $ 551,019
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense 139,481      
Liability for Claims and Claims Adjustment Expense 3,893,868 $ 3,771,240 $ 3,691,719 $ 3,517,728
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 1,175,230      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 181,102      
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 330,467      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 21,898      
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 873,882      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 220,683      
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 532,320      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 15,641      
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 85,744      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 3,473      
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 58,980      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 12,620      
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 13,604      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 367      
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 9,122      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 2,909      
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 2,735,278      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 436,428      
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 96,070      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 45,572      
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 31,106      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 1,346      
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 10,474      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 31,777      
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 137,650      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 78,695      
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 344,071      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments $ 22,265