Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Loss and Loss Adjustment Expense Reserve
Gross beginning balance	$ 3,159.8	$ 3,079.3
Less beginning reinsurance recoverable on unpaid losses	(483.9)	(428.1)
Net loss and LAE reserves	2,675.9	2,651.2
Loss and LAE reserves acquired		13.6
Loss and LAE reserves consolidated — SSIE	0
Loss and LAE incurred relating to current year losses	266.8	240.5
Net (favorable) and unfavorable loss reserve development	(1.4)	(11.2)
Total incurred losses and LAE	265.4	229.3
Foreign currency translation adjustment to net loss and LAE reserves	(22.4)	1.8
Current year losses	(40.6)	(35.0)
Prior year losses	(280.2)	(237.7)
Total loss and LAE payments	(320.8)	(272.7)
Net ending balance	2,598.1	2,623.2
Plus ending reinsurance recoverable on unpaid losses	436.9	425.2
Gross ending balance	$ 3,035.0	$ 3,048.4