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, 2013		Dec. 31, 2012		Dec. 31, 2011
Gross
Beginning balance	$ 1,162.4		$ 1,354.1		$ 1,023.0
Incurred losses and LAE	0		(1.2)		488.6
Paid losses and LAE	(148.6)		(190.5)		(157.5)
Ending balance	1,013.8		1,162.4		1,354.1
Beginning balance	727.9	[1]	832.8	[1]	741.1	[1]
Incurred losses and LAE	0	[1]	(1.0)	[1]	94.4	[1]
Paid losses and LAE	(112.3)	[1]	(103.9)	[1]	(2.7)	[1]
Ending balance	615.6	[1]	727.9	[1]	832.8	[1]
Beginning balance	8.8	[2]	11.2	[2]	15.6	[2]
Incurred losses and LAE	0	[2]	(1.0)	[2]	6.0	[2]
Liability for unpaid claims and claims adjustment expense	(3.1)	[2]	(1.4)	[2]	(10.4)	[2]
Ending balance	5.7	[2]	8.8	[2]	11.2	[2]
Asbestos
Gross
Beginning balance	929.4		1,074.3		904.0
Incurred losses and LAE	0		(0.3)		256.8
Paid losses and LAE	(84.3)		(144.6)		(86.5)
Ending balance	845.1		929.4		1,074.3
Beginning balance	602.5	[1]	681.2	[1]	647.3	[1]
Incurred losses and LAE	0	[1]	(0.5)	[1]	32.2	[1]
Paid losses and LAE	(79.3)	[1]	(78.2)	[1]	1.7	[1]
Ending balance	523.2	[1]	602.5	[1]	681.2	[1]
Beginning balance	2.4	[2]	2.2	[2]	6.4	[2]
Incurred losses and LAE	0	[2]	(0.5)	[2]	(4.0)	[2]
Liability for unpaid claims and claims adjustment expense	(0.5)	[2]	0.7	[2]	(0.2)	[2]
Ending balance	1.9	[2]	2.4	[2]	2.2	[2]
Environmental
Gross
Beginning balance	233.0		279.8		119.0
Incurred losses and LAE	0		(0.9)		231.8
Paid losses and LAE	(64.3)		(45.9)		(71.0)
Ending balance	168.7		233.0		279.8
Beginning balance	125.4	[1]	151.6	[1]	93.8	[1]
Incurred losses and LAE	0	[1]	(0.5)	[1]	62.2	[1]
Paid losses and LAE	(33.0)	[1]	(25.7)	[1]	(4.4)	[1]
Ending balance	92.4	[1]	125.4	[1]	151.6	[1]
Beginning balance	6.4	[2]	9.0	[2]	9.2	[2]
Incurred losses and LAE	0	[2]	(0.5)	[2]	10.0	[2]
Liability for unpaid claims and claims adjustment expense	(2.6)	[2]	(2.1)	[2]	(10.2)	[2]
Ending balance	$ 3.8	[2]	$ 6.4	[2]	$ 9.0	[2]

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

[2] Includes NICO cover