Third Party Reinsurance (Sirius Group) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Reinsurance [Line Items]
Reinsurance recoverable on paid losses	$ 14.8			$ 30.5
Reinsurance recoverable on unpaid losses	356.8		2,369.4	2,507.3	2,581.2	2,193.0	2,344.0
Seller Note From Acquisition	25.0
OneBeacon
Reinsurance [Line Items]
Reinsurance recoverable on paid losses	2.0
Reinsurance recoverable on unpaid losses	41.3
Reinsurance Recoverables	1,449.2
Number of ratings in A.M. Best ratings	15
Sirius
Reinsurance [Line Items]
Reinsurance recoverable on paid losses	12.7
Reinsurance recoverable on unpaid losses	315.5
General Reinsurance Corporation [Member] | Sirius
Reinsurance [Line Items]
Reinsurance Recoverables	42.3
Percentage of total paid and unpaid reinsurance recoverables	13.00%
Percentage Collateralized	1.00%
Swiss Re Group [Member] | Sirius
Reinsurance [Line Items]
Reinsurance Recoverables	33.7
Percentage of total paid and unpaid reinsurance recoverables	10.00%
Percentage Collateralized	5.00%
Olympus [Member] | Sirius
Reinsurance [Line Items]
Reinsurance Recoverables	28.8	[1]
Percentage of total paid and unpaid reinsurance recoverables	9.00%	[1]
Percentage Collateralized	100.00%	[1]
Lloyds of London [Member] | Sirius
Reinsurance [Line Items]
Reinsurance Recoverables	29.9	[2]
Percentage of total paid and unpaid reinsurance recoverables	9.00%	[2]
Percentage Collateralized	4.00%	[2]
Michigan Catastorphic Claims Association [Member] | Sirius
Reinsurance [Line Items]
Reinsurance Recoverables	$ 14.6
Percentage of total paid and unpaid reinsurance recoverables	4.00%
Percentage Collateralized	0.00%

[1] (4) Michigan Catastrophic Claims Association ("MCCA") is a non-profit unincorporated association., established by the State of Michigan with the power to issue and collect assessments, to which every insurance company that sells automobile coverage in Michigan is required to be a member. A.M. Best does not rate MCCA. Sirius Group acquired its recoverable from MCCA in the acquisition of Stockbridge Insurance Company. As part of the acquisition, Sirius Group obtained $25.0 of reinsurance protection from the seller (currently rated A+ by A.M. Best) for unfavorable loss reserve development, including uncollectible reinsurance.

[2] (3) Non-U.S. insurance entity. The balance is fully collateralized through funds held, letters of credit or trust agreements.