Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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	$ 5.2		$ 3.7
Unfavorable adjustment to prior years liability for unpaid claims and claims adjustment expense		10.0
Causes of increase (decrease) in liability for unpaid claims and claims adjustment expense, amount	(5.2)
Loss Adjustment Expenses Recognized In 2009 [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			2.5
Loss Adjustment Expenses Recognized In 2010 [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			0.8
Unfavorable adjustment to prior years liability for unpaid claims and claims adjustment expense		3.3
Loss Adjustment Expenses Recognized In 2011 [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	2.4		0.4
Unfavorable adjustment to prior years liability for unpaid claims and claims adjustment expense		1.7
Loss Adjustment Expenses Recognized In 2012 [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	4.4	5.0
Causes of increase (decrease) in liability for unpaid claims and claims adjustment expense, amount			(3.7)
Loss Adjustment Expenses Recognized In 2013 [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	7.2
Causes of increase (decrease) in liability for unpaid claims and claims adjustment expense, amount		10.0
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		3.7
Causes of increase (decrease) in liability for unpaid claims and claims adjustment expense, amount	(4.1)		(3.7)
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		1.5
Causes of increase (decrease) in liability for unpaid claims and claims adjustment expense, amount	(1.9)
E & S Commercial Business Unit [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	4.7
Causes of increase (decrease) in liability for unpaid claims and claims adjustment expense, amount			(0.3)
E & S Commercial Business Unit [Member] | Loss Adjustment Expenses Recognized In 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Causes of increase (decrease) in liability for unpaid claims and claims adjustment expense, amount		16.0
Hallmark Select 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		2.6
Causes of increase (decrease) in liability for unpaid claims and claims adjustment expense, amount	(1.0)		(3.3)
Hallmark Select Business Unit [Member] | Loss Adjustment Expenses Recognized In 2011 [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		2.3
Commerical Excess Liability [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	0.9
Medical Professional Liability Products [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	0.4
General Aviation [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	0.3
Personal Lines Business Unit [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			3.6
Causes of increase (decrease) in liability for unpaid claims and claims adjustment expense, amount	(2.9)
Personal Lines Business Unit [Member] | Loss Adjustment Expenses Recognized In 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Causes of increase (decrease) in liability for unpaid claims and claims adjustment expense, amount		1.8
Excess and Umbrella 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		0.4
Aircraft Hull [Member] | Loss Adjustment Expenses Recognized In 2012 [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		$ 0.1