Third Party Reinsurance (Sirius Group) (Details) (USD $) In Millions, unless otherwise specified						3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2013 OneBeacon Rating	Mar. 31, 2013 Sirius Rating	Mar. 31, 2013 Berkshire Hathaway [Member] Sirius	Mar. 31, 2013 Swiss Re Group Sirius	Mar. 31, 2013 Olympus Sirius		Mar. 31, 2013 Lloyds of London Sirius		Mar. 31, 2013 Michigan Catastrophic Claims Association Sirius
Reinsurance [Line Items]
Reinsurance recoverable on paid losses	$ 15.6	$ 17.9			$ 2.4	$ 13.2
Reinsurance recoverable on unpaid losses	410.8	429.1	2,465.4	2,507.3	105.8	305.0
Reinsurance recoverables							56.9	34.8	24.9	[1]	17.0	[2]	14.7	[3]
Percentage of total paid and unpaid reinsurance recoverables							18.00%	11.00%	8.00%	[1]	5.00%	[2]	5.00%	[3]
Percentage Collateralized							0.00%	0.00%	100.00%	[1]	9.00%	[2]	0.00%	[3]
Number of ratings in A.M. Best ratings					16	16
Seller Note From Acquisition						$ 25.0

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

[2] Represents the total of reinsurance recoverables due to Sirius Group from all Lloyds Syndicates.

[3] 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.