Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Summary) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	$ 1,389.8	$ 1,342.2	$ 1,054.3
Less beginning reinsurance recoverable on unpaid losses	(186.0)	(161.6)	(80.2)
Net loss and LAE reserves	1,203.8	1,180.6	974.1
Current Accident Year	649.2	712.9	732.0
Prior Accident Year	14.8	(4.0)	92.0
Total incurred losses and LAE	664.0	708.9	824.0
Loss and LAE paid relating to current year losses	(192.8)	(208.8)	(202.6)
Loss and LAE paid relating to prior year losses	(483.1)	(479.1)	(420.8)
Total loss and LAE payments	(675.9)	(687.9)	(623.4)
Net loss and LAE reserves	1,192.7	1,203.8	1,180.6
Plus ending reinsurance recoverable on unpaid losses	172.9	186.0	161.6
Gross ending balance	1,365.6	1,389.8	1,342.2
Star & Shield Insurance Exchange [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net loss and LAE reserves	5.5	7.7
Net loss and LAE reserves	$ 4.7	$ 5.5	7.7
Star & Shield Insurance Exchange [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net loss and LAE reserves			$ 13.6