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Derivatives (Value, Collateral (held) provided by WM Life Re and Net Exposure to Credit Losses on OTC Derivative Instruments) (Details) - White Mountains Life Re - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Offsetting Liabilities [Line Items] |  |
Derivative Assets | $ 16.7 | $ 23.2
Derivative Liabilities | (3.6) | (3.1)
Net amounts recognized in Other assets | 13.1 | 20.1
Over The Counter | Fixed income/interest rate |  |
Offsetting Liabilities [Line Items] |  |
Derivative Assets | 2.2 | 2.4
Derivative Liabilities | (2.5) | (2.1)
Net amounts recognized in Other assets | (0.3) | 0.3
Over The Counter | Foreign exchange |  |
Offsetting Liabilities [Line Items] |  |
Derivative Assets | 12.7 | 15.0
Derivative Liabilities | 0.0 | 0.0
Net amounts recognized in Other assets | 12.7 | 15.0
Over The Counter | Equity |  |
Offsetting Liabilities [Line Items] |  |
Derivative Assets | 1.5 | 4.4
Derivative Liabilities | (0.9) | (0.6)
Net amounts recognized in Other assets | 0.6 | 3.8
Exchange Traded | Fixed income/interest rate |  |
Offsetting Liabilities [Line Items] |  |
Derivative Assets | 0.0 | 0.1
Derivative Liabilities | 0.0 | (0.1)
Net amounts recognized in Other assets | 0.0 | 0.0
Exchange Traded | Foreign exchange |  |
Offsetting Liabilities [Line Items] |  |
Derivative Assets | 0.0 | 0.1
Derivative Liabilities | (0.1) | (0.3)
Net amounts recognized in Other assets | (0.1) | (0.2)
Exchange Traded | Equity |  |
Offsetting Liabilities [Line Items] |  |
Derivative Assets | 0.3 | 1.2
Derivative Liabilities | (0.1) | 0.0
Net amounts recognized in Other assets | $ 0.2 | $ 1.2

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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: 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 3: 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=49121117&loc=d3e13433-108611 Reference 4: 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 5: 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=49121117&loc=d3e13495-108611 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313 + Details
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