SCHEDULE IV REINSURANCE (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
OneBeacon
Premiums earned
Gross amount	$ 1,138.5	$ 1,732.5	$ 2,122.4
Ceded to other companies	(166.1)	(309.0)	(229.7)
Assumed from other companies	43.1	64.2	66.8
Net amount	1,015.5	1,487.7	1,959.5
Percent of amount assumed to net	4.20%	4.30%	3.40%
Sirius group
Premiums earned
Gross amount	128.5	117.9	107.2
Ceded to other companies	(205.9)	(199.7)	(175.9)
Assumed from other companies	989.8	929.7	927.5
Net amount	$ 912.4	$ 847.9	$ 858.8
Percent of amount assumed to net	108.50%	109.60%	108.00%

Amount of premiums assumed from other companies.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 235

-Section S99

-Paragraph 2

-Subparagraph (SX 210.12-17)

-URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e574992-122915

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