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Variable Annuity Reinsurance (Details) | 3 Months Ended |  |  | 3 Months Ended |  |  |  |  |  |  |
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Mar. 31, 2012 USD ($) | Mar. 31, 2011 USD ($) | Mar. 31, 2012 JPY (¥) | Dec. 31, 2011 USD ($) | Dec. 31, 2011 JPY (¥) | Mar. 31, 2012 Fixed income/Interest rate USD ($) | Mar. 31, 2011 Fixed income/Interest rate USD ($) | Mar. 31, 2012 Foreign exchange USD ($) | Mar. 31, 2011 Foreign exchange USD ($) | Mar. 31, 2012 Equity USD ($) | Mar. 31, 2011 Equity USD ($) | Mar. 31, 2012 Level 1 Inputs USD ($) | Mar. 31, 2011 Level 1 Inputs USD ($) | Mar. 31, 2012 Level 2 Inputs USD ($) | Mar. 31, 2011 Level 2 Inputs USD ($) | Mar. 31, 2012 Level 3 Inputs USD ($) | Mar. 31, 2011 Level 3 Inputs USD ($) | Mar. 31, 2012 Fair Value USD ($) | Mar. 31, 2011 Fair Value USD ($) | Mar. 31, 2012 WM Life Re USD ($) rating | Mar. 31, 2011 WM Life Re USD ($) | Dec. 31, 2011 WM Life Re USD ($) | Mar. 31, 2012 WM Life Re Citigroup USD ($) | Mar. 31, 2012 WM Life Re Royal Bank of Scotland USD ($) | Mar. 31, 2012 WM Life Re Bank of America USD ($) | Mar. 31, 2012 WM Life Re JP Morgan USD ($) | Mar. 31, 2012 WM Life Re Nomura USD ($) | Mar. 31, 2012 WM Life Re Barclays USD ($) | Mar. 31, 2012 WM Life Re Goldman Sachs USD ($) | Mar. 31, 2012 WM Life Re Cash USD ($) | Dec. 31, 2011 WM Life Re Cash USD ($) | Mar. 31, 2012 WM Life Re Fixed maturity investments. USD ($) | Dec. 31, 2011 WM Life Re Fixed maturity investments. USD ($) | Mar. 31, 2012 WM Life Re Short-term investments USD ($) | Dec. 31, 2011 WM Life Re Short-term investments USD ($)
Variable annuity reinsurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total guarantee value | $ 2,800,000,000 |  | ¥ 232,700,000,000 | $ 3,000,000,000 | ¥ 233,700,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collective account values of underlying variable annuities as a percentage of the guarantee value | 84.00% |  |  | 78.00% | 78.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax operating results of WM Life Re |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fees, included in other revenues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,000,000 | 7,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of variable annuity liability, included in other revenues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 211,200,000 | 52,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of derivatives, included in other revenues | (206,500,000) | (65,500,000) |  |  |  | (79,000,000) | (6,200,000) | (55,000,000) | (47,400,000) | (72,500,000) | (11,900,000) | (48,000,000) | (19,600,000) | (98,200,000) | (20,700,000) | (60,300,000) | (25,200,000) | (206,500,000) | (65,500,000) | (206,500,000) | (65,500,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Foreign exchange, included in other revenues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (23,000,000) | (4,600,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other investment income and gain (loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (2,100,000) | (400,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenues | 630,600,000 | 529,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (12,400,000) | (10,300,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of variable annuity death benefit liabilities, included in other expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,800,000 | 3,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Death benefit claims paid, included in other expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (1,800,000) | (1,000,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
General and administrative expenses | (45,500,000) | (28,000,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (1,300,000) | (700,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax loss | 128,300,000 | (33,700,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (6,700,000) | (9,000,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in variable annuity liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance of liabilities at the beginning of the period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (768,500,000) | (610,200,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Realized and unrealized gains (losses) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 220,000,000 | 55,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance of liabilities at the end of the period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (548,500,000) | (554,900,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in derivative instruments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance of assets at the beginning of the period | 290,400,000 |  |  |  |  | 31,100,000 |  | 161,300,000 |  | 98,000,000 |  | 4,100,000 |  | 39,200,000 | 72,200,000 | 247,100,000 | 275,300,000 | 290,400,000 | 347,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6,100,000 | 5,000,000 | 6,100,000 | 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Realized and unrealized gains (losses) | (206,500,000) | (65,500,000) |  |  |  | (79,000,000) | (6,200,000) | (55,000,000) | (47,400,000) | (72,500,000) | (11,900,000) | (48,000,000) | (19,600,000) | (98,200,000) | (20,700,000) | (60,300,000) | (25,200,000) | (206,500,000) | (65,500,000) | (206,500,000) | (65,500,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sales/settlements |  |  |  |  |  |  |  |  |  |  |  | 34,500,000 | 26,900,000 | 58,900,000 | 3,300,000 | (9,100,000) | (29,100,000) | 84,300,000 | 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance of assets at the end of the period | 174,300,000 |  |  |  |  | 4,000,000 |  | 109,500,000 |  | 60,800,000 |  | (9,400,000) | 7,300,000 | (100,000) | 54,800,000 | 183,800,000 | 226,000,000 | 174,300,000 | 288,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues | (206,500,000) | (65,500,000) |  |  |  | (79,000,000) | (6,200,000) | (55,000,000) | (47,400,000) | (72,500,000) | (11,900,000) | (48,000,000) | (19,600,000) | (98,200,000) | (20,700,000) | (60,300,000) | (25,200,000) | (206,500,000) | (65,500,000) | (206,500,000) | (65,500,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value | 174,300,000 |  |  |  |  | 4,000,000 |  | 109,500,000 |  | 60,800,000 |  | (9,400,000) | 7,300,000 | (100,000) | 54,800,000 | 183,800,000 | 226,000,000 | 174,300,000 | 288,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit risk on derivative instruments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OTC derivative instruments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 190,700,000 |  | 295,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Collateral held |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (43,900,000) |  | (73,200,000) |  |  |  |  |  |  |  | 360,100,000 | 453,500,000 | 29,300,000 | 31,200,000 | 1,500,000 | 600,000
Collateral provided |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 115,400,000 |  | 83,000,000 |  |  |  |  |  |  |  |  |  |  |  | 43,900,000 | 73,200,000
Cash and short-term investments held and posted as collateral to its reinsurance counterparties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 390,900,000 | 301,300,000 | 485,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Net exposure to credit losses on fair value of OTC instruments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 262,200,000 |  | 305,200,000 | 63,400,000 | 59,800,000 | 52,800,000 | 35,800,000 | 28,500,000 | 17,900,000 | 4,000,000 |  |  |  |  |  |
Number of creditworthiness ratings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustments for counterparty credit risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (7,000,000) |  | $ (9,100,000) |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Net increase or decrease in the fair value of the variable annuity death benefit liabilities included in earnings. + References No definition available. + Details
Name: | wtm_GainLossOnLiabilityForDeathBenefit
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Net increase or decrease in the fair value of the variable annuity liabilities included in earnings. + References No definition available. + Details
Name: | wtm_GainLossOnVariableAnnuityLiabilities
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that were not covered by collateral. + References No definition available. + Details
Name: | wtm_NetExposureOnFairValueOfOverTheCounterDerivativeInstruments
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Represents the number of creditworthiness ratings used by the entity. + References No definition available. + Details
Name: | wtm_NumberOfCreditRatings
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition This item represents other investment income and gains (losses) included in earnings for the period. + References No definition available. + Details
Name: | wtm_OtherInvestmentIncomeAndGainLossOnInvestments
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Total nominal account value guaranteed with certain variable annuities. + References No definition available. + Details
Name: | wtm_VariableAnnuityNominalGuaranteeValue
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | wtm_VariableAnnuityReinsuranceLineItems
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration