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Fair Value of Financial Instruments (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior Notes	$ 669.0	$ 818.8
WMRe Preference Shares	250.0	250.0
OneBeacon U.S. Holdings, Inc. ("OBH") | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior Notes	269.7	419.6
Carrying Value | White Mountains Re.
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior Notes	399.3	399.2
WMRe Preference Shares	250.0	250.0
OneBeacon U.S. Holdings, Inc. ("OBH") | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior Notes	282.2	435.1
Fair Value | White Mountains Re.
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior Notes	414.7	390.9
WMRe Preference Shares	$ 237.5	$ 212.5

X

- Definition

Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13,16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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X

- Definition

Represents preference shares issued by a subsidiary, White Mountains Re Group Ltd. Amount is classified as equity not attributable to White Mountains (noncontrolling interest, minority interest).

+ Details

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