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Derivatives (Realized and Unrealized Gains (Losses) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2013 | Mar. 31, 2012 | Dec. 31, 2012
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |
Realized and unrealized (losses) gains | $ (181.8) | $ (206.5) |
Derivative, fair value, net | 75.8 |  | 98.3
Fixed income/interest rate |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |
Realized and unrealized (losses) gains | (30.9) | (79.0) |
Derivative, fair value, net | (16.2) |  | 27.1
Foreign exchange |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |
Realized and unrealized (losses) gains | (53.6) | (55.0) |
Derivative, fair value, net | 74.1 |  | 52.8
Equity |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |
Realized and unrealized (losses) gains | (97.3) | (72.5) |
Derivative, fair value, net | $ 17.9 |  | $ 18.4

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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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