Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable adjustment to prior years liability for unpaid claims and claims adjustment expense		$ 7,000	$ 5,200
Unfavorable adjustment to prior years liability for unpaid claims and claims adjustment expense	$ 7,600
Claims and Claim Adjustment Expenses Incurred Related to Prior Years	7,608	(6,953)	(5,203)
Loss Adjustment Expenses Recognized in 2013 and Prior [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	9,000
Loss Adjustment Expenses Recognized In 2014 [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	3,900
Loss Adjustment Expenses Recognized in 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable adjustment to prior years liability for unpaid claims and claims adjustment expense	5,300
Specialty Commercial Segment [Member] | Loss Adjustment Expenses Recognized In 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and Claim Adjustment Expenses Incurred Related to Prior Years	138,842	133,464	144,996
Specialty Commercial Segment [Member] | Loss Adjustment Expenses Recognized in 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and Claim Adjustment Expenses Incurred Related to Prior Years	146,610	147,304
Specialty Commercial Unit [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and Claim Adjustment Expenses Incurred Related to Prior Years	1,200
Medical Professional Liability Products [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	900
Commercial Auto [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	700
General Aviation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable adjustment to prior years liability for unpaid claims and claims adjustment expense	300
Commercial Umbrella and Excess Liability Risks [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable adjustment to prior years liability for unpaid claims and claims adjustment expense	100
MGA Commercial Products [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and Claim Adjustment Expenses Incurred Related to Prior Years	11,300
Standard Commercial Segment [Member] | Loss Adjustment Expenses Recognized In 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and Claim Adjustment Expenses Incurred Related to Prior Years	53,568	55,808	55,488
Standard Commercial Segment [Member] | Loss Adjustment Expenses Recognized in 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and Claim Adjustment Expenses Incurred Related to Prior Years	49,857	49,571
Standard Commercial P & C Business Unit [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable adjustment to prior years liability for unpaid claims and claims adjustment expense	6,600
Workers Compensation Business Unit [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable adjustment to prior years liability for unpaid claims and claims adjustment expense	3,300
Personal Segment [Member] | Loss Adjustment Expenses Recognized In 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and Claim Adjustment Expenses Incurred Related to Prior Years	6,243	5,340	$ 5,452
Personal Segment [Member] | Loss Adjustment Expenses Recognized in 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and Claim Adjustment Expenses Incurred Related to Prior Years	25,682	$ 23,104
Specialty Personal [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and Claim Adjustment Expenses Incurred Related to Prior Years	$ 5,000