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Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | Sep. 30, 2014 Rating | Dec. 31, 2013
--- | --- | ---
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | $ 65.3 | $ 69.2
Collertal provided to counterparty in cash | 0.3 | 0
Collateral provided to counterparty in financial instruments | 0 | 0.40
Net amount of exposure after effect of collateral | 65.6 | 69.6
Excess collateral provided to counterparty in cash | 23.4 | 44.1
Derivative, Collateral, Right to Reclaim Securities | 10.4 | 22.8
Counter-party collateral held by WMLife Re - Cash | 0 | (0.8)
Net amount of exposure to counter-party | 99.4 | 135.7
S and P Credit Ratings | 21 |
Standard & Poor's, A Rating | Bank of America [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 16.5 | 27.2
Collertal provided to counterparty in cash | 0 | 0
Collateral provided to counterparty in financial instruments | 0 | 0
Net amount of exposure after effect of collateral | 16.5 | 27.2
Excess collateral provided to counterparty in cash | 0 | 0
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Counter-party collateral held by WMLife Re - Cash | 0 | 0
Net amount of exposure to counter-party | 16.5 | 27.2
Standard & Poor's, A Rating | Barclays |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 0.5 | 1.4
Collertal provided to counterparty in cash | 0 | 0
Collateral provided to counterparty in financial instruments | 0 | 0
Net amount of exposure after effect of collateral | 0.5 | 1.4
Excess collateral provided to counterparty in cash | 0 | 0
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Counter-party collateral held by WMLife Re - Cash | 0 | 0
Net amount of exposure to counter-party | 0.5 | 1.4
Standard & Poor's, A Rating | Royal Bank of Scotland |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments |  | 11.3
Collertal provided to counterparty in cash |  | 0
Collateral provided to counterparty in financial instruments |  | 0
Net amount of exposure after effect of collateral |  | 11.3
Excess collateral provided to counterparty in cash |  | 0
Derivative, Collateral, Right to Reclaim Securities |  | 0
Counter-party collateral held by WMLife Re - Cash |  | 0
Net amount of exposure to counter-party |  | 11.3
Standard & Poor's, A Rating | Citigroup - OTC |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 15.5 | 19.4
Collertal provided to counterparty in cash | 0 | 0
Collateral provided to counterparty in financial instruments | 0 | 0
Net amount of exposure after effect of collateral | 15.5 | 19.4
Excess collateral provided to counterparty in cash | 6.9 | 2.3
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Counter-party collateral held by WMLife Re - Cash | 0 | 0
Net amount of exposure to counter-party | 22.4 | 21.7
Standard & Poor's, A Rating | Citigroup Exchange Traded [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 4.3 | 1.2
Collertal provided to counterparty in cash | 0 | 0
Collateral provided to counterparty in financial instruments | 0 | 0
Net amount of exposure after effect of collateral | 4.3 | 1.2
Excess collateral provided to counterparty in cash | 13.9 | 19.8
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Counter-party collateral held by WMLife Re - Cash | 0 | 0
Net amount of exposure to counter-party | 18.2 | 21.0
Standard & Poor's, A plus | JP Morgan |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 21.7 | 9.1
Collertal provided to counterparty in cash | 0 | 0
Collateral provided to counterparty in financial instruments | 0 | 0
Net amount of exposure after effect of collateral | 21.7 | 9.1
Excess collateral provided to counterparty in cash | 0 | 22.0
Derivative, Collateral, Right to Reclaim Securities | 0 | 0
Counter-party collateral held by WMLife Re - Cash | 0 | 0
Net amount of exposure to counter-party | 21.7 | 31.1
Standard & Poor's, BBB+ Rating [Member] | Nomura [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | (0.3) | (0.4)
Collertal provided to counterparty in cash | 0.3 | 0
Collateral provided to counterparty in financial instruments | 0 | 0.40
Net amount of exposure after effect of collateral | 0 | 0
Excess collateral provided to counterparty in cash | 2.6 | 0
Derivative, Collateral, Right to Reclaim Securities | 10.4 | 22.8
Counter-party collateral held by WMLife Re - Cash | 0 | (0.8)
Net amount of exposure to counter-party | 13.0 | 22.0
Standard & Poor's, A- Rating | Royal Bank of Scotland |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 7.1 |
Collertal provided to counterparty in cash | 0 |
Collateral provided to counterparty in financial instruments | 0 |
Net amount of exposure after effect of collateral | 7.1 |
Excess collateral provided to counterparty in cash | 0 |
Derivative, Collateral, Right to Reclaim Securities | 0 |
Counter-party collateral held by WMLife Re - Cash | 0 |
Net amount of exposure to counter-party | $ 7.1 |

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- Definition Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=28370219&loc=SL20226016-175313 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=20225523&loc=SL20225862-175312 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959 + Details
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