Reserves for Losses and Loss Adjustment Expenses (Reconciliation of Incurred and Paid Claims to Liability for Unpaid Losses and LAE) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated loss adjustment expense, net of reinsurance	$ 431,821	$ 340,997
Reinsurance recoverable on unpaid claims	349,585	272,604
Unallocated claims adjustment expenses	8,362	6,754
Total gross liability for unpaid claims and claim adjustment expenses	789,768	620,355	$ 527,247
Commercial Auto business units [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated loss adjustment expense, net of reinsurance	218,999	189,896
Reinsurance recoverable on unpaid claims	91,040	112,931
Unallocated claims adjustment expenses	1,850	1,383
E&S Casualty business unit [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated loss adjustment expense, net of reinsurance	97,302	63,528
Reinsurance recoverable on unpaid claims	122,341	91,900
Unallocated claims adjustment expenses	1,681	760
Commercial Accounts business unit [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated loss adjustment expense, net of reinsurance	59,603	51,200
Reinsurance recoverable on unpaid claims	20,464	13,671
Unallocated claims adjustment expenses	2,643	2,732
Aviation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated loss adjustment expense, net of reinsurance	2,448	2,514
Reinsurance recoverable on unpaid claims	10,143	9,469
Unallocated claims adjustment expenses	104	85
Runoff [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated loss adjustment expense, net of reinsurance	2,226	3,109
Reinsurance recoverable on unpaid claims	1,184	1,326
Unallocated claims adjustment expenses	170	170
Specialty Commercial Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated loss adjustment expense, net of reinsurance	5,644	4,234
Reinsurance recoverable on unpaid claims	968	3,618
Unallocated claims adjustment expenses	61	63
Personal Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated loss adjustment expense, net of reinsurance	18,597	15,027
Reinsurance recoverable on unpaid claims	5,613	11,752
Unallocated claims adjustment expenses	1,250	1,250
Property [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated loss adjustment expense, net of reinsurance	27,002	11,489
Reinsurance recoverable on unpaid claims	97,832	27,937
Unallocated claims adjustment expenses	$ 603	$ 311