Reserves for Unpaid Losses and Loss Adjustment Expenses (Causes for Prior Accident Year Reserve Development by Segment) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable net prior year development	$ 4,453	$ 9,684
Standard Commercial Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable net prior year development	(1,560)	264
Specialty Commercial Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable net prior year development	6,861	8,332
Personal Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
(Favorable) unfavorable net prior year development	$ (848)	$ 1,088