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SCHEDULE IV REINSURANCE | 12 Months Ended
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Dec. 31, 2012
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract] |
SCHEDULE IV REINSURANCE | WHITE MOUNTAINS INSURANCE GROUP, LTD. REINSURANCE (Millions)
Column A |  | Column B |  | Column C |  | Column D |  | Column E |  | Column F
Premiums earned |  | Gross amount |  | Ceded to other companies |  | Assumed from other companies |  | Net amount |  | Percentage of amount assumed to net
Years ended: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
December 31, 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OneBeacon |  | $ | 1,158.3 |  |  | $ | (79.1 | ) |  | $ | 52.8 |  |  | $ | 1,132.0 |  |  | 4.7 | %
Sirius Group |  | 169.9 |  |  | (226.6 | ) |  | 988.3 |  |  | 931.6 |  |  | 106.1 | %
December 31, 2011 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OneBeacon |  | $ | 1,035.9 |  |  | $ | (66.0 | ) |  | $ | 42.3 |  |  | $ | 1,012.2 |  |  | 4.2 | %
Sirius Group |  | 128.5 |  |  | (206.0 | ) |  | 989.8 |  |  | 912.3 |  |  | 108.5 | %
December 31, 2010 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OneBeacon |  | $ | 1,242.5 |  |  | $ | (122.1 | ) |  | $ | 60.7 |  |  | $ | 1,181.1 |  |  | 5.1 | %
Sirius Group |  | 117.9 |  |  | (199.7 | ) |  | 929.7 |  |  | 847.9 |  |  | 109.6 | %

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- Definition The entire disclosure for the supplemental schedule of reinsurance information required to be filed for each period for which an audited income statement is presented. Reinsurance schedule includes the type of insurance, gross amount of premiums, premiums ceded to other companies, premiums assumed from other companies, net amount of premium revenue, and the percentage of the premiums assumed to net premium revenue. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 05 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 3: 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 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.7-05.(c) Schedule IV) -URI http://asc.fasb.org/extlink&oid=6879921&loc=d3e569147-122900 + Details
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