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Derivatives (Realized and Unrealized Gains (Losses) (Details) (White Mountains Life Re [Member], USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended |
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Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011 | Dec. 31, 2011
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |
Derivative, Gain (Loss) on Derivative, Net | $ (11.0) | $ 148.9 | $ (122.0) | $ 105.4 |
Derivative, Fair Value, Net | 242.1 |  | 242.1 |  | 290.4
Interest Rate Contract [Member] |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |
Derivative, Gain (Loss) on Derivative, Net | (22.6) | 29.8 | (73.1) | 17.3 |
Derivative, Fair Value, Net | 29.1 |  | 29.1 |  | 31.1
Foreign Exchange Contract [Member] |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |
Derivative, Gain (Loss) on Derivative, Net | 6.5 | 63.0 | (21.0) | 32.8 |
Derivative, Fair Value, Net | 127.9 |  | 127.9 |  | 161.3
Equity Contract [Member] |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |
Derivative, Gain (Loss) on Derivative, Net | 5.1 | 56.1 | (27.9) | 55.3 |
Derivative, Fair Value, Net | $ 85.1 |  | $ 85.1 |  | $ 98.0

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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 Net Increase or Decrease in the fair value of the derivative or group of derivatives included in earnings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959 + Details
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