Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Gross beginning balance	$ 1,365.6	$ 1,389.8
Less beginning reinsurance recoverable on unpaid losses	(172.9)	(186.0)
Net loss and LAE reserves	1,192.7	1,203.8
Loss and LAE incurred relating to current year losses	151.7	161.2
Net (favorable) and unfavorable loss reserve development	0.0	0.1
Total incurred losses and LAE	151.7	161.1
Current year losses	(21.5)	(23.1)
Prior year losses	(129.1)	(148.6)
Total loss and LAE payments	(150.6)	(171.7)
Net ending balance	1,194.1	1,193.4
Plus ending reinsurance recoverable on unpaid losses	174.7	150.4
Gross ending balance	1,368.8	1,343.8
Star & Shield Insurance Exchange
Net ending balance	0.0
Star & Shield Insurance Exchange
Net (favorable) and unfavorable loss reserve development		0.1
Star & Shield Insurance Exchange
Net loss and LAE reserves	4.7	5.5
Net ending balance	$ 4.4	$ 5.3