Reserves for Losses and Loss Adjustment Expenses (Causes for Prior Accident Year Reserve Development by Segment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total unfavorable (favorable) net prior year development	$ 58,288	$ 60,900
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	45,808	60,138
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	3,357	726
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	9,123	36
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