Reserves for Unpaid Losses and Loss Adjustment Expenses (Causes for Prior Accident Year Reserve Development by Segment) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total unfavorable (favorable) net prior year development	$ 33,259	$ 7,781
Specialty Commercial Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total unfavorable (favorable) net prior year development	23,961	11,232
Standard Commercial Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total unfavorable (favorable) net prior year development	2,350	(3,508)
Personal Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total unfavorable (favorable) net prior year development	6,948	57
Corporate [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total unfavorable (favorable) net prior year development	$ 0	$ 0