Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2012		Dec. 31, 2011
Loss and Loss Adjustment Expense Reserve
Gross beginning balance	$ 3,057.9	$ 5,329.8	$ 3,168.9	$ 5,702.3
Less beginning reinsurance recoverable on unpaid losses	(388.6)	(2,369.4)	(429.1)	(2,507.3)
Net loss and LAE reserves	2,692.1	2,703.0	2,692.1	2,703.0	2,669.3		2,739.8		2,960.4		3,195.0
Loss and LAE reserves acquired - American Fuji	0	0	21.3	0
Loss and LAE incurred relating to current year losses	289.9	318.2	822.8	842.9
Net favorable and unfavorable loss reserve development	(11.6)	(10.1)	(25.6)	(21.2)
Total incurred losses and LAE	278.3	308.1	797.2	821.7
Accretion of fair value adjustment to net loss and LAE reserves	0.2	1.1	1.5	9.4
Foreign currency translation adjustment to net loss and LAE reserves	12.3	12.0	(1.9)	11.7
Current year losses	(90.0)	(101.1)	(209.9)	(223.9)
Prior year losses	(178.0)	(181.5)	(655.9)	(726.8)
Total loss and LAE payments	(268.0)	(282.6)	(865.8)	(950.7)
Net loss and LAE reserves	2,692.1	2,703.0	2,692.1	2,703.0	2,669.3		2,739.8		2,960.4		3,195.0
Reinsurance recoverable on unpaid losses	416.0	356.8	416.0	356.8
Gross ending balance	3,108.1	3,059.8	3,108.1	3,059.8
OneBeacon Runoff
Loss and Loss Adjustment Expense Reserve
Net loss and LAE reserves					0	[1]	0	[1]	(296.0)	[1]	(384.1)	[1]
Net loss and LAE reserves					$ 0	[1]	$ 0	[1]	$ (296.0)	[1]	$ (384.1)	[1]

[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.