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Debt (Details) (USD $) In Millions, unless otherwise specified | Sep. 30, 2012 | Dec. 31, 2011 | Sep. 30, 2012 OBH Senior Notes | Dec. 31, 2011 OBH Senior Notes | Sep. 30, 2012 SIG Senior Notes | Dec. 31, 2011 SIG Senior Notes | Sep. 30, 2012 WTM Bank Facility | Dec. 31, 2011 WTM Bank Facility | Aug. 12, 2011 WTM Bank Facility | Sep. 30, 2012 Old Lyme | Dec. 31, 2011 Old Lyme | Dec. 30, 2011 Old Lyme Y | Sep. 30, 2012 Other debt | Dec. 31, 2011 Other debt
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Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument at face value |  |  | $ 269.9 | $ 269.9 | $ 400.0 | $ 400.0 |  |  |  |  |  |  |  |  |  |
Unamortized original issue discount |  |  | (0.1) | (0.1) | (0.6) | (0.7) |  |  |  |  |  |  |  |  |  |
Total debt | 676.6 | 677.5 | 269.8 | 269.8 | 399.4 | 399.3 | 0 | 0 |  | 2.1 | 2.1 |  | 5.3 | [1] | 6.3 | [1]
Total commitment under revolving credit facility |  |  |  |  |  |  |  |  | 375.0 |  |  |  |  |  |  |
Debt Instrument Term (in years) |  |  |  |  |  |  |  |  |  |  |  | 5 |  |  |  |
Percentage of principal amount of debt subject to upward or downward adjustments |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |
Cash to be paid on acquisition |  |  |  |  |  |  |  |  |  |  |  | $ 6.0 |  |  |  |
[1] | (1) Other debt relates to White Mountains’ consolidation of Hamer and Bri-Mar.

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- Definition The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 21 -Paragraph 16, 20 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number APB14-1 -Paragraph 31 -Subparagraph 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 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400 + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 129 -Paragraph 2, 4 -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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