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Segment Information (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2010 | Sep. 30, 2010 | Jun. 30, 2010 | Mar. 31, 2010 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2008
Segment reporting information |  |  |  |  |  |  |  |  |  |  |  |
Earned insurance and reinsurance premiums |  |  |  |  |  |  |  |  | $ 1,927.8 | $ 2,251.8 | $ 2,717.6 |
Net investment income |  |  |  |  |  |  |  |  | 184.5 | 208.9 | 250.1 |
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  | 122.8 | 82.6 | 361.7 |
Other revenue |  |  |  |  |  |  |  |  | (57.0) | 23.4 | 113.9 |
Total revenues | 593.6 | 501.3 | 553.6 | 529.6 | 552.8 | 551.0 | 670.8 | 792.1 | 2,178.1 | 2,566.7 | 3,443.3 |
Losses and LAE |  |  |  |  |  |  |  |  | 1,206.9 | 1,389.2 | 1,424.1 |
Insurance and reinsurance acquisition expenses |  |  |  |  |  |  |  |  | 400.0 | 479.1 | 547.2 |
Other underwriting expenses |  |  |  |  |  |  |  |  | 279.1 | 332.9 | 406.6 |
General and administrative expenses |  |  |  |  |  |  |  |  | 175.3 | 155.1 | 197.4 |
Interest expense on debt |  |  |  |  |  |  |  |  | 55.2 | 57.3 | 70.8 |
Total expenses | 535.6 | 500.1 | 517.5 | 563.3 | 457.2 | 475.8 | 664.9 | 815.7 | 2,116.5 | 2,413.6 | 2,646.1 |
Pre-tax income | 58.0 | 1.2 | 36.1 | (33.7) | 95.6 | 75.2 | 5.9 | (23.6) | 61.6 | 153.1 | 797.2 |
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |  |
Total investments | 8,268.0 |  |  |  | 7,943.5 |  |  |  | 8,268.0 | 7,943.5 |  |
Reinsurance recoverable on paid and unpaid losses | 2,537.8 |  |  |  | 2,407.1 |  |  |  | 2,537.8 | 2,407.1 |  |
Assets held for sale | 132.6 |  |  |  | 1,122.8 |  |  |  | 132.6 | 1,122.8 |  |
Total assets | 14,064.0 |  |  |  | 14,534.1 |  |  |  | 14,064.0 | 14,534.1 |  |
Loss and LAE reserves | 5,702.3 |  |  |  | 5,736.8 |  |  |  | 5,702.3 | 5,736.8 | 6,379.2 | 7,029.4
Liabilities held for sale | 107.6 |  |  |  | 751.5 |  |  |  | 107.6 | 751.5 |  |
Total liabilities | 9,396.1 |  |  |  | 10,273.3 |  |  |  | 9,396.1 | 10,273.3 |  |
Noncontrolling interest | 580.2 |  |  |  | 607.8 |  |  |  | 580.2 | 607.8 |  |
Total White Mountains' common shareholders' equity | 4,087.7 |  |  |  | 3,653.0 |  |  |  | 4,087.7 | 3,653.0 |  |
OneBeacon |  |  |  |  |  |  |  |  |  |  |  |
Segment reporting information |  |  |  |  |  |  |  |  |  |  |  |
Number of transactions completed, which transformed the reportable segment into a specialty insurance company |  |  |  |  |  |  |  |  | 2 |  |  |
Earned insurance and reinsurance premiums |  |  |  |  |  |  |  |  | 1,015.5 | 1,403.9 | 1,858.8 |
Net investment income |  |  |  |  |  |  |  |  | 71.4 | 96.6 | 125.5 |
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  | 10.6 | 74.6 | 248.6 |
Other revenue |  |  |  |  |  |  |  |  | (10.7) | 9.6 | 23.1 |
Total revenues |  |  |  |  |  |  |  |  | 1,086.8 | 1,584.7 | 2,256.0 |
Losses and LAE |  |  |  |  |  |  |  |  | 580.9 | 858.2 | 1,005.3 |
Insurance and reinsurance acquisition expenses |  |  |  |  |  |  |  |  | 219.0 | 311.6 | 376.3 |
Other underwriting expenses |  |  |  |  |  |  |  |  | 173.3 | 233.1 | 308.5 |
General and administrative expenses |  |  |  |  |  |  |  |  | 9.8 | 13.0 | 18.5 |
Interest expense on debt |  |  |  |  |  |  |  |  | 20.5 | 29.6 | 39.7 |
Total expenses |  |  |  |  |  |  |  |  | 1,003.5 | 1,445.5 | 1,748.3 |
Pre-tax income |  |  |  |  |  |  |  |  | 83.3 | 139.2 | 507.7 |
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |  |
Total investments | 2,707.6 |  |  |  | 3,266.0 |  |  |  | 2,707.6 | 3,266.0 |  |
Reinsurance recoverable on paid and unpaid losses | 2,184.1 |  |  |  | 1,937.7 |  |  |  | 2,184.1 | 1,937.7 |  |
Assets held for sale | 132.6 |  |  |  |  |  |  |  | 132.6 |  |  |
Total assets | 5,792.4 |  |  |  | 6,138.3 |  |  |  | 5,792.4 | 6,138.3 |  |
Loss and LAE reserves | 3,358.6 |  |  |  | 3,295.5 |  |  |  | 3,358.6 | 3,295.5 |  |
Liabilities held for sale | 107.6 |  |  |  |  |  |  |  | 107.6 |  |  |
Total liabilities | 4,678.5 |  |  |  | 4,889.4 |  |  |  | 4,678.5 | 4,889.4 |  |
Noncontrolling interest | 287.2 |  |  |  | 314.9 |  |  |  | 287.2 | 314.9 |  |
Total White Mountains' common shareholders' equity | 826.7 |  |  |  | 934.0 |  |  |  | 826.7 | 934.0 |  |
Sirius |  |  |  |  |  |  |  |  |  |  |  |
Segment reporting information |  |  |  |  |  |  |  |  |  |  |  |
Earned insurance and reinsurance premiums |  |  |  |  |  |  |  |  | 912.3 | 847.9 | 858.8 |
Net investment income |  |  |  |  |  |  |  |  | 89.9 | 96.5 | 110.3 |
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  | 101.9 | (9.8) | 112.6 |
Other revenue |  |  |  |  |  |  |  |  | (44.6) | 35.9 | 106.5 |
Total revenues |  |  |  |  |  |  |  |  | 1,059.5 | 970.5 | 1,188.2 |
Losses and LAE |  |  |  |  |  |  |  |  | 626.0 | 531.0 | 418.8 |
Insurance and reinsurance acquisition expenses |  |  |  |  |  |  |  |  | 181.0 | 167.5 | 170.9 |
Other underwriting expenses |  |  |  |  |  |  |  |  | 105.8 | 99.8 | 98.1 |
General and administrative expenses |  |  |  |  |  |  |  |  | 34.1 | 31.6 | 74.4 |
Interest expense on debt |  |  |  |  |  |  |  |  | 31.6 | 26.6 | 26.3 |
Total expenses |  |  |  |  |  |  |  |  | 978.5 | 856.5 | 788.5 |
Pre-tax income |  |  |  |  |  |  |  |  | 81.0 | 114.0 | 399.7 |
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |  |
Total investments | 3,640.5 |  |  |  | 4,116.2 |  |  |  | 3,640.5 | 4,116.2 |  |
Reinsurance recoverable on paid and unpaid losses | 353.7 |  |  |  | 469.4 |  |  |  | 353.7 | 469.4 |  |
Total assets | 5,337.9 |  |  |  | 5,618.7 |  |  |  | 5,337.9 | 5,618.7 |  |
Loss and LAE reserves | 2,343.7 |  |  |  | 2,441.3 |  |  |  | 2,343.7 | 2,441.3 |  |
Total liabilities | 3,784.1 |  |  |  | 3,863.0 |  |  |  | 3,784.1 | 3,863.0 |  |
Noncontrolling interest | 289.5 |  |  |  | 290.6 |  |  |  | 289.5 | 290.6 |  |
Total White Mountains' common shareholders' equity | 1,264.3 |  |  |  | 1,465.1 |  |  |  | 1,264.3 | 1,465.1 |  |
Other Operations |  |  |  |  |  |  |  |  |  |  |  |
Segment reporting information |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  |  |  |  |  |  |  | 23.2 | 15.8 | 14.3 |
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  | 10.3 | 17.8 | 0.5 |
Other revenue |  |  |  |  |  |  |  |  | (1.7) | (22.1) | (15.7) |
Total revenues |  |  |  |  |  |  |  |  | 31.8 | 11.5 | (0.9) |
General and administrative expenses |  |  |  |  |  |  |  |  | 131.4 | 110.5 | 104.5 |
Interest expense on debt |  |  |  |  |  |  |  |  | 3.1 | 1.1 | 4.8 |
Total expenses |  |  |  |  |  |  |  |  | 134.5 | 111.6 | 109.3 |
Pre-tax income |  |  |  |  |  |  |  |  | (102.7) | (100.1) | (110.2) |
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |  |
Total investments | 1,919.9 |  |  |  | 561.3 |  |  |  | 1,919.9 | 561.3 |  |
Total assets | 2,933.7 |  |  |  | 1,654.3 |  |  |  | 2,933.7 | 1,654.3 |  |
Total liabilities | 933.5 |  |  |  | 769.4 |  |  |  | 933.5 | 769.4 |  |
Noncontrolling interest | 3.5 |  |  |  | 2.3 |  |  |  | 3.5 | 2.3 |  |
Total White Mountains' common shareholders' equity | 1,996.7 |  |  |  | 882.6 |  |  |  | 1,996.7 | 882.6 |  |
Discontinued Operations. |  |  |  |  |  |  |  |  |  |  |  |
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |  |
Assets held for sale |  |  |  |  | 1,122.8 |  |  |  |  | 1,122.8 |  |
Total assets |  |  |  |  | 1,122.8 |  |  |  |  | 1,122.8 |  |
Liabilities held for sale |  |  |  |  | 751.5 |  |  |  |  | 751.5 |  |
Total liabilities |  |  |  |  | 751.5 |  |  |  |  | 751.5 |  |
Total White Mountains' common shareholders' equity |  |  |  |  | $ 371.3 |  |  |  |  | $ 371.3 |  |

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- Definition Premiums earned on the income statement for all insurance and reinsurance contracts after subtracting any amounts ceded to another insurer and adding premiums assumed from other insurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(c)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -Subparagraph a, b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 + Details
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- Definition The known and estimated amount recoverable as of the balance sheet date from reinsurers for claims paid or incurred by the ceding insurer and associated claims settlement expenses, including estimated amounts for claims incurred but not reported, and policy benefits, net of any related valuation allowance. + 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.(a),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 6 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 14, 16, 27, 113 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 + Details
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- Definition Alternate concept name for the aggregate amount of policy reserves (provided for future obligations including unpaid claims and claims adjustment expenses) and policy benefits (liability for future policy benefits) as of the balance sheet date; grouped amount of all the liabilities associated with the company's insurance policies. + 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 210.7-03.13(a)(1)) -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 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 5 -Paragraph 9 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 13 -Subparagraph a + Details
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- Definition Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 5 -Section 03 -Paragraph 1 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688 + Details
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- Definition Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29-31) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph A3 -Appendix A Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30, 31 -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 4 -Section E Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB TOPIC 4.E) -URI http://asc.fasb.org/extlink&oid=6228006&loc=d3e74512-122707 + Details
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- Definition Assets related to a disposition that have been classified as Held for Sale. + References No definition available. + Details
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- Definition Amount of policy acquisition costs charged to expense in the period. Includes deferred policy acquisition costs amortized in the period, generally in proportion to related revenue earned over the policy or contract term. + References No definition available. + Details
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- Definition Represents the number of transactions completed which resulted in transformation of the segment into a specialty insurance company. + References No definition available. + Details
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