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Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2013 | Dec. 31, 2012
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White Mountains Life Re |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets | $ 69.2 | [1] | $ 98.3 | [1]
Credit risk on derivative instruments |  |  |  |
Net exposure on fair value of OTC instruments | 135.7 |  | 222.1 |
Derivative, Collateral, Right to Reclaim Cash | 0 |  | 45.3 |
Derivative, Collateral, Right to Reclaim Securities | 0.40 |  | 0 |
Derivative Asset, Fair Value, Amount Offset Against Collateral | 69.6 |  | 143.6 |
Excess collateral provided to counterparty in cash | 44.1 |  | 81.1 |
Excess collateral providedto counterparty in financial instruments | 22.8 |  | 28.0 |
Derivative, Collateral, Obligation to Return Cash | 0.8 |  | 30.6 |
Forward contracts |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets | (0.1) |  | (0.1) |
Credit risk on derivative instruments |  |  |  |
S&P Credit Ratings | 21 |  |  |
A | Citigroup OTC [Member] | White Mountains Life Re |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets | 19.4 |  | 19.9 |
Credit risk on derivative instruments |  |  |  |
Net exposure on fair value of OTC instruments | 21.7 |  | 50.7 |
Derivative, Collateral, Right to Reclaim Cash | 0 |  | 0 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Derivative Asset, Fair Value, Amount Offset Against Collateral | 19.4 |  | 19.9 |
Excess collateral provided to counterparty in cash | 2.3 |  | 30.8 |
Excess collateral providedto counterparty in financial instruments | 0 |  | 0 |
Derivative, Collateral, Obligation to Return Cash | 0 |  | 0 |
A | Royal Bank of Scotland [Member] | White Mountains Life Re |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets | 11.3 |  | 33.6 |
Credit risk on derivative instruments |  |  |  |
Net exposure on fair value of OTC instruments | 11.3 |  | 33.6 |
Derivative, Collateral, Right to Reclaim Cash | 0 |  | 0 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Derivative Asset, Fair Value, Amount Offset Against Collateral | 11.3 |  | 33.6 |
Excess collateral provided to counterparty in cash | 0 |  | 0 |
Excess collateral providedto counterparty in financial instruments | 0 |  | 0 |
Derivative, Collateral, Obligation to Return Cash | 0 |  | 0 |
A | Barclays [Member] | White Mountains Life Re |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets | 1.4 |  |  |
Credit risk on derivative instruments |  |  |  |
Net exposure on fair value of OTC instruments | 1.4 |  |  |
Derivative, Collateral, Right to Reclaim Cash | 0 |  |  |
Derivative, Collateral, Right to Reclaim Securities | 0 |  |  |
Derivative Asset, Fair Value, Amount Offset Against Collateral | 1.4 |  |  |
Excess collateral provided to counterparty in cash | 0 |  |  |
Excess collateral providedto counterparty in financial instruments | 0 |  |  |
Derivative, Collateral, Obligation to Return Cash | 0 |  |  |
A | Bank of America [Member] | White Mountains Life Re |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets | 27.2 |  | 78.5 |
Credit risk on derivative instruments |  |  |  |
Net exposure on fair value of OTC instruments | 27.2 |  | 47.9 |
Derivative, Collateral, Right to Reclaim Cash | 0 |  | 0 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Derivative Asset, Fair Value, Amount Offset Against Collateral | 27.2 |  | 78.5 |
Excess collateral provided to counterparty in cash | 0 |  | 0 |
Excess collateral providedto counterparty in financial instruments | 0 |  | 0 |
Derivative, Collateral, Obligation to Return Cash | 0 |  | 30.6 |
A | Citigroup Exchange Traded [Member] | White Mountains Life Re |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets | 1.2 |  | (21.5) |
Credit risk on derivative instruments |  |  |  |
Net exposure on fair value of OTC instruments | 21.0 |  | 13.6 |
Derivative, Collateral, Right to Reclaim Cash | 0 |  | 21.5 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Derivative Asset, Fair Value, Amount Offset Against Collateral | 1.2 |  | 0 |
Excess collateral provided to counterparty in cash | 19.8 |  | 13.6 |
Excess collateral providedto counterparty in financial instruments | 0 |  | 0 |
Derivative, Collateral, Obligation to Return Cash | 0 |  | 0 |
BBB | Nomura [Member] | White Mountains Life Re |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets | (0.4) |  | (0.9) |
Credit risk on derivative instruments |  |  |  |
Net exposure on fair value of OTC instruments | 22.0 |  | 28.8 |
Derivative, Collateral, Right to Reclaim Cash | 0 |  | 0.9 |
Derivative, Collateral, Right to Reclaim Securities | 0.40 |  | 0 |
Derivative Asset, Fair Value, Amount Offset Against Collateral | 0 |  | 0 |
Excess collateral provided to counterparty in cash | 0 |  | 0.8 |
Excess collateral providedto counterparty in financial instruments | 22.8 |  | 28.0 |
Derivative, Collateral, Obligation to Return Cash | 0.8 |  | 0 |
Standard & Poor's, A+ Rating [Member] | J P Morgan Chase [Member] | White Mountains Life Re |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets | 9.1 |  | (22.8) |
Credit risk on derivative instruments |  |  |  |
Net exposure on fair value of OTC instruments | 31.1 |  | 32.8 |
Derivative, Collateral, Right to Reclaim Cash | 0 |  | 22.8 |
Derivative, Collateral, Right to Reclaim Securities | 0 |  | 0 |
Derivative Asset, Fair Value, Amount Offset Against Collateral | 9.1 |  | 0 |
Excess collateral provided to counterparty in cash | 22.0 |  | 32.8 |
Excess collateral providedto counterparty in financial instruments | 0 |  | 0 |
Derivative, Collateral, Obligation to Return Cash | 0 |  | 0 |
Standard & Poor's, A+ Rating [Member] | Barclays [Member] | White Mountains Life Re |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets |  |  | 11.6 |
Credit risk on derivative instruments |  |  |  |
Net exposure on fair value of OTC instruments |  |  | 11.6 |
Derivative, Collateral, Right to Reclaim Cash |  |  | 0 |
Derivative, Collateral, Right to Reclaim Securities |  |  | 0 |
Derivative Asset, Fair Value, Amount Offset Against Collateral |  |  | 11.6 |
Excess collateral provided to counterparty in cash |  |  | 0 |
Excess collateral providedto counterparty in financial instruments |  |  | 0 |
Derivative, Collateral, Obligation to Return Cash |  |  | 0 |
Standard & Poor's, A- Rating [Member] | Goldman Sachs [Member] | White Mountains Life Re |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |
Net amounts recognized in Other Assets |  |  | (0.1) |
Credit risk on derivative instruments |  |  |  |
Net exposure on fair value of OTC instruments |  |  | 3.1 |
Derivative, Collateral, Right to Reclaim Cash |  |  | 0.1 |
Derivative, Collateral, Right to Reclaim Securities |  |  | 0 |
Derivative Asset, Fair Value, Amount Offset Against Collateral |  |  | 0 |
Excess collateral provided to counterparty in cash |  |  | 3.1 |
Excess collateral providedto counterparty in financial instruments |  |  | 0 |
Derivative, Collateral, Obligation to Return Cash |  |  | $ 0 |
[1] | All derivative instruments held by WM Life Re are subject to master netting arrangements.

X
- Definition Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets. + 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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- 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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