Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 OneBeacon Runoff		Dec. 31, 2012 OneBeacon Runoff		Mar. 31, 2012 OneBeacon Runoff		Dec. 31, 2011 OneBeacon Runoff
Loss and Loss Adjustment Expense Reserve
Gross beginning balance	$ 3,168.9	$ 5,702.3
Less beginning reinsurance recoverable on unpaid losses	(429.1)	(2,507.3)
Net loss and LAE reserves	2,739.8	3,195.0	0	[1]	0	[1]	332.4	[1]	383.3	[1]
Loss and LAE reserves acquired - American Fuji	21.3	0
Loss and LAE incurred relating to current year losses	242.9	257.4
Net favorable and unfavorable loss reserve development	1.4	(7.6)
Total incurred losses and LAE	244.3	249.8
Accretion of fair value adjustment to net loss and LAE reserves	1.2	7.1
Foreign currency translation adjustment to net loss and LAE reserves	(9.6)	11.8
Current year losses	(36.6)	(44.5)
Prior year losses	(270.6)	(322.9)
Total loss and LAE payments	(307.2)	(367.4)
Net loss and LAE reserves	2,689.8	3,045.4	0	[1]	0	[1]	332.4	[1]	383.3	[1]
Plus ending reinsurance recoverable on unpaid losses	410.8	2,465.4
Gross ending balance	$ 3,100.6	$ 5,510.8

[1] Loss and LAE reserve balances for OneBeacon's run-off business prior to September 30, 2012 were not classified as held for sale. Adjustment is to present loss and LAE reserve activities from continuing operations.