Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Loss and Loss Adjustment Expense Reserve
Gross beginning balance	$ 3,035.0	$ 3,048.4	$ 3,159.8	$ 3,079.3
Less beginning reinsurance recoverable on unpaid losses	(436.9)	(425.2)	(483.9)	(428.1)
Net loss and LAE reserves	2,598.1	2,623.2	2,675.9	2,651.2
Loss and LAE reserves acquired				13.6
Loss and LAE reserves consolidated — SSIE	0.0	0.0	0.0
Loss and LAE incurred relating to current year losses	300.0	278.4	566.8	518.9
Net (favorable) and unfavorable loss reserve development	(11.7)	4.3	(13.1)	(6.9)
Total incurred losses and LAE	288.3	282.7	553.7	512.0
Foreign currency translation adjustment to net loss and LAE reserves	7.5	(0.5)	(14.9)	1.3
Current year losses	(72.9)	(65.8)	(113.5)	(100.8)
Prior year losses	(216.7)	(219.5)	(496.9)	(457.2)
Total loss and LAE payments	(289.6)	(285.3)	(610.4)	(558.0)
Net ending balance	2,604.3	2,620.1	2,604.3	2,620.1
Plus ending reinsurance recoverable on unpaid losses	427.5	433.2	427.5	433.2
Gross ending balance	$ 3,031.8	$ 3,053.3	$ 3,031.8	$ 3,053.3