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Third Party Reinsurance (One Beacon) (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
Reinsurance [Line Items]
Reinsurance recoverable on paid losses	$ 15.6	$ 17.9			$ 2.4
Reinsurance recoverable on unpaid losses	410.8	429.1	2,465.4	2,507.3	105.8
Reinsurance Recoverables on Unpaid Losses					90.00%
Number of ratings in A.M. Best ratings					16
Largest Recoverable from reinsurer					$ 11.0

The portion of reinsurance receivables representing the aggregate amount of estimated amounts due as of the balance sheet date from another insurer; also called reinsurance recoverables or unpaid losses recoverable, that are expected based upon statistical projections of incurred claims and claims settlement expenses which have not yet been reported to the insurer.

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- Definition

Receivables currently due from reinsurers for ceded claims paid.

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