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Reserve for Loss and Loss Expense (Reconciliation of Claims Development to Liability) (Details) - USD ($)
$ in Thousands
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net $ 4,566,037      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 618,601 $ 757,513 $ 578,641 $ 554,269
Unallocated loss expenses 152,273      
Gross liability for unpaid loss and loss expenses 5,336,911 $ 5,144,821 $ 4,580,903 $ 4,260,355
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 1,874,944      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 255,493      
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 767,784      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 203,982      
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 939,644      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 15,683      
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 101,601      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 5,252      
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 133,412      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 39,152      
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 18,637      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 10,241      
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 3,836,022      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 529,803      
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 125,940      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 38,273      
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 58,334      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 1,647      
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 14,178      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 32,736      
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 198,452      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 72,656      
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 531,563      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 16,142      
E&S Lines [Member] | Standard Commercial Lines [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments 456      
E&S Lines [Member] | Property Insurance [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 13,575      
E&S Lines [Member] | Casualty Insurance Product Line [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for Unpaid Claims and Claims Adjustment Expense, Net 517,988      
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments $ 15,686