Reserves for Unpaid Losses and Loss Adjustment Expenses (Impact from Unfavorable Prior Years' Loss Development by Reporting Segment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable (Favorable) Adjustment To Prior Years Liability For Unpaid Claims And Claims Adjustment Expense	$ 5,953	$ 40,105
Specialty Commercial Segment Member
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable (Favorable) Adjustment To Prior Years Liability For Unpaid Claims And Claims Adjustment Expense	16,457	40,477
Standard Commercial Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable (Favorable) Adjustment To Prior Years Liability For Unpaid Claims And Claims Adjustment Expense	(8,993)	(970)
Personal Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable (Favorable) Adjustment To Prior Years Liability For Unpaid Claims And Claims Adjustment Expense	$ (1,511)	$ 598