Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Summary) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	$ 3,079.3	$ 3,168.9	$ 5,702.3
Less beginning reinsurance recoverable on unpaid losses	(428.1)	(429.1)	(2,507.3)
Net loss and LAE reserves	2,651.2	2,739.8	3,195.0
Current Accident Year	1,175.3	1,088.9	1,235.8
Prior Accident Year	(6.0)	(48.4)	(41.9)
Total incurred losses and LAE	1,169.3	1,040.5	1,193.9
Accretion of fair value adjustment to net loss and LAE reserves	0.7	1.7	10.6
Foreign currency translation adjustment to net loss and LAE reserves	(36.0)	0.3	12.9
Loss and LAE paid relating to current year losses	(345.3)	(336.2)	(404.7)
Loss and LAE paid relating to prior year losses	(823.0)	(832.6)	(901.6)
Total loss and LAE payments	(1,168.3)	(1,168.8)	(1,306.3)
Net loss and LAE reserves	2,675.9	2,651.2	2,739.8
Plus ending reinsurance recoverable on unpaid losses	483.9	428.1	429.1
Gross ending balance	3,159.8	3,079.3	3,168.9
AutoOne
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net loss and LAE reserves				0	(383.3)
Net loss and LAE reserves		0		0	(383.3)
Sirius Group
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Loss and LAE reserves acquired	45.4	37.7	17.0
Star & Shield Insurance Exchange [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Loss and LAE reserves consolidated — SSIE	$ 13.6	$ 0	$ 0