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Reserve for Loss and Loss Expense (Reconciliation of Claims Development to Liability) (Details) - USD ($)
$ in Thousands
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net $ 3,873,898      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 578,641 $ 554,269 [1] $ 544,162 $ 537,388
Unallocated loss expenses 128,364      
Gross liability for unpaid loss and loss expenses 4,580,903 $ 4,260,355 $ 4,067,163 $ 3,893,868
Standard Commercial Lines [Member] | General Liability [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 1,515,789      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 213,253      
Standard Commercial Lines [Member] | Workers Compensation [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 816,606      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 196,670      
Standard Commercial Lines [Member] | Commercial Automobile [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 717,704      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 15,480      
Standard Commercial Lines [Member] | Business Owners' Policies [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 99,501      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 6,828      
Standard Commercial Lines [Member] | Commercial Property [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 98,204      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 22,277      
Standard Commercial Lines [Member] | Other Commercial [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 22,866      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 2,136      
Standard Commercial Lines [Member] | Total 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 3,270,670      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 456,644      
Standard Personal Lines | Personal Automobile [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 98,476      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 40,941      
Standard Personal Lines | Homeowners [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 44,328      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 2,392      
Standard Personal Lines | Other Personal [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 12,261      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 64,975      
Standard Personal Lines | Standard Personal Lines        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 155,065      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 108,308      
E&S Lines [Member] | General Liability [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 430,176      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 11,672      
E&S Lines [Member] | Total 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 448,163      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 13,689      
E&S 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 17,987      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments $ 2,017      
[1] ncludes an adjustment of $2.9 million related to our adoption of ASU 2016-13, Financial Instruments - Credit Losses.