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Derivatives (Forward Contracts) (Details) In Millions, unless otherwise specified | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |
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Dec. 31, 2012 Forward contracts USD ($) Rating | Dec. 31, 2012 Forward contracts SEK | Dec. 31, 2012 Forward contracts USD USD ($) | Dec. 31, 2012 Forward contracts SEK USD ($) | Dec. 31, 2012 Forward contracts EUR USD ($) | Dec. 31, 2012 Forward contracts GBP USD ($) | Dec. 31, 2012 Forward contracts Currency Translation USD ($) | Dec. 31, 2012 Forward contracts Deutsche Bank USD ($) | Dec. 31, 2012 Forward contracts Barclays Bank London USD ($) | Dec. 31, 2012 Forward contracts HSBC Bank plc USD ($) | Dec. 31, 2012 Forward contracts JP Morgan USD ($) | Dec. 31, 2012 Forward contracts Goldman Sachs USD ($) | Dec. 31, 2012 White Mountains Life Re USD ($) | Dec. 31, 2011 White Mountains Life Re USD ($) | Dec. 31, 2010 White Mountains Life Re USD ($) | Dec. 31, 2012 White Mountains Life Re JP Morgan USD ($) | Dec. 31, 2012 White Mountains Life Re Goldman Sachs USD ($)
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount | $ 31.2 | 202.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
S&P Credit Ratings | 21 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in derivative instruments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning of period | 0 |  |  |  |  |  |  |  |  |  |  |  | 290.4 |  |  |  |  |
Purchases | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of derivatives, included in other revenues | (0.3) |  | (0.2) | 0 | (0.1) | 0 | 0 |  |  |  |  |  | (339.0) | 92.9 | 127.0 |  |  |
End of period | 0.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of derivatives, included in other revenues | (0.3) |  | (0.2) | 0 | (0.1) | 0 | 0 |  |  |  |  |  | (339.0) | 92.9 | 127.0 |  |  |
Derivative, Fair Value, Net | (0.1) |  | 0 | 0 | (0.1) | 0 | 0 |  |  |  |  |  | 98.3 | 290.4 |  |  |  |
Net exposure on fair value of OTC instruments | 31.2 |  |  |  |  |  |  | 11.1 | 7.7 | 10.1 | 1.9 | 0.4 | 212.2 | 305.2 |  | 30.9 | [1] | 3.1
Uncollaterailized balance | $ (0.1) |  |  |  |  |  |  | $ 0 | $ (0.1) | $ 0 | $ 0 | $ 0 |  |  |  |  |  |
[1] | Collateral provided (held) calculated under master netting agreement.

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- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13495-108611 + Details
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- Definition Purchases that have taken place during the period in relation to derivative assets measured at fair value on a recurring basis. + References No definition available. + Details
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