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, 2018	Sep. 30, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) net prior years' development	$ 6,062	$ 20,240
Specialty Commercial Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) net prior years' development	15,730	17,824
Standard Commercial Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) net prior years' development	(8,829)	1,594
Personal Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable (favorable) net prior years' development	$ (839)	$ 822