Reserve for Loss and Loss Expense (Reconciliation of Claims Development to Liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 3,576,212
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	554,269	$ 544,162	$ 537,388	$ 585,855
Unallocated loss expenses	129,874
Gross liability for unpaid loss and loss expenses	4,260,355	$ 4,067,163	$ 3,893,868	$ 3,771,240
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	1,380,902
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	215,136
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	828,146
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	210,450
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	630,512
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	11,611
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	91,419
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	6,849
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	74,182
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	21,760
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	16,359
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	2,853
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	3,021,520
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	468,659
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	94,237
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	42,403
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	44,159
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	847
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	10,751
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	29,589
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	149,147
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	72,839
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	389,460
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	12,195
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	405,545
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	12,771
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	16,085
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	$ 576