Discontinued Operations Discontinued Operations (Asbestos and environmental loss and LAE reserve activity) (Details) (OneBeacon Runoff, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Gross
Beginning balance	$ 1,354.1		$ 1,023.0		$ 1,336.3
Incurred losses and LAE	(1.2)		488.6		6.2
Paid losses and LAE	(190.5)		(157.5)		(319.5)
Ending balance	1,162.4		1,354.1		1,023.0
Beginning balance	832.8	[1]	741.1	[1]	907.4	[1]
Incurred losses and LAE	(1.0)	[1]	94.4	[1]	6.0	[1]
Paid losses and LAE	(103.9)	[1]	(2.7)	[1]	(172.3)	[1]
Ending balance	727.9	[1]	832.8	[1]	741.1	[1]
Beginning balance	11.2	[2]	15.6	[2]	14.1	[2]
Incurred losses and LAE	(1.0)	[2]	6.0	[2]	6.0	[2]
Liability for unpaid claims and claims adjustment expense	1.4	[2]	10.4	[2]	4.5	[2]
Ending balance	8.8	[2]	11.2	[2]	15.6	[2]
Asbestos Issue
Gross
Beginning balance	1,074.3		904.0		985.6
Incurred losses and LAE	(0.3)		256.8		0
Paid losses and LAE	(144.6)		(86.5)		(81.6)
Ending balance	929.4		1,074.3		904.0
Beginning balance	681.2	[1]	647.3	[1]	688.8	[1]
Incurred losses and LAE	(0.5)	[1]	32.2	[1]	0	[1]
Paid losses and LAE	(78.2)	[1]	1.7	[1]	(41.5)	[1]
Ending balance	602.5	[1]	681.2	[1]	647.3	[1]
Beginning balance	2.2	[2]	6.4	[2]	6.5	[2]
Incurred losses and LAE	(0.5)	[2]	(4.0)	[2]	0	[2]
Liability for unpaid claims and claims adjustment expense	(0.7)	[2]	0.2	[2]	0.1	[2]
Ending balance	2.4	[2]	2.2	[2]	6.4	[2]
Environmental Issue
Gross
Beginning balance	279.8		119.0		350.7
Incurred losses and LAE	(0.9)		231.8		6.2
Paid losses and LAE	(45.9)		(71.0)		(237.9)
Ending balance	233.0		279.8		119.0
Beginning balance	151.6	[1]	93.8	[1]	218.6	[1]
Incurred losses and LAE	(0.5)	[1]	62.2	[1]	6.0	[1]
Paid losses and LAE	(25.7)	[1]	(4.4)	[1]	(130.8)	[1]
Ending balance	125.4	[1]	151.6	[1]	93.8	[1]
Beginning balance	9.0	[2]	9.2	[2]	7.6	[2]
Incurred losses and LAE	(0.5)	[2]	10.0	[2]	6.0	[2]
Liability for unpaid claims and claims adjustment expense	2.1	[2]	10.2	[2]	4.4	[2]
Ending balance	$ 6.4	[2]	$ 9.0	[2]	$ 9.2	[2]

[1] Represents A&E reserve activity, net of third-party reinsurance, but prior to the NICO Cover.

[2] Includes NICO cover