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, 2017	Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable Adjustment To Prior Years Liability For Unpaid Claims and Claims Adjustment Expense	$ 40,105	$ 7,608
Specialty Commercial Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable Adjustment To Prior Years Liability For Unpaid Claims and Claims Adjustment Expense	40,477	12,502
Standard Commercial Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable Adjustment To Prior Years Liability For Unpaid Claims and Claims Adjustment Expense	(970)	(9,901)
Personal Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable Adjustment To Prior Years Liability For Unpaid Claims and Claims Adjustment Expense	$ 598	$ 5,007