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Municipal Bond Guarantee Insurance (Details) $ in Millions | 1 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Apr. 30, 2018 USD ($) | Dec. 31, 2019 USD ($) Contract | Dec. 31, 2018 USD ($) Contract | Dec. 31, 2017 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2020 | Sep. 30, 2019 USD ($) | Jul. 31, 2012 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured |  | 15.00% |  |  |  |  |  |
Percentage of risk premium ceded |  |  | 60.00% |  |  |  |  |
Assets Held-in-trust |  | $ 786.7 | $ 757.4 |  |  |  |  |
Interest receivable |  | $ 162.7 | $ 143.7 |  |  |  |  |
HG Re |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured |  |  | 15.00% |  |  |  |  |
White Mountains |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Capital contributions |  |  |  |  | $ 594.5 |  |  |
HG Global |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  | 503.0 |  |  |
Percent over a target trust balance |  | 102.00% |  |  |  |  |  |
Cash and fixed income securities |  |  |  |  |  |  |  | $ 100.0
Supplement Trust target |  | $ 603.0 |  |  |  |  |  |
BAM |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Contracts outstanding | Contract |  | 8,987 | 7,525 |  |  |  |  |
Remaining weighted average contract period (in years) |  | 10 years 8 months 12 days |  |  |  |  |  |
Municipal bond outstanding principal |  | $ 62,250.5 | $ 52,201.6 |  |  |  | $ 1,100.0 |
Municipal bond outstanding interest |  | 31,799.7 | 26,560.3 |  |  |  |  |
Municipal bond debt service outstanding |  | 94,050.2 | 78,761.9 |  |  |  |  |
Unearned premium revenue |  | 198.4 | $ 176.0 |  |  |  |  |
January 1, 2020 - March 31, 2020 |  | 4.8 |  |  |  |  |  |
April 1, 2020 - June 30, 2020 |  | 4.8 |  |  |  |  |  |
July 1, 2020 - September 30, 2020 |  | 4.6 |  |  |  |  |  |
October 1, 2020 - December 31, 2020 |  | 4.5 |  |  |  |  |  |
2020 |  | 18.7 |  |  |  |  |  |
2021 |  | 17.3 |  |  |  |  |  |
2022 |  | 16.3 |  |  |  |  |  |
2023 |  | 15.2 |  |  |  |  |  |
2024 |  | 14.2 |  |  |  |  |  |
2025 and thereafter |  | $ 116.7 |  |  |  |  |  |
BAM Management |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Capital contributions |  |  |  |  | $ 14.5 |  |  |
Preferred stocks | HG Global |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  | 96.90% | 96.90% |  |  |  |  |
Common Stock | HG Global |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  | 88.40% | 88.40% |  |  |  |  |
BAM |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Surplus notes |  | $ 457.6 | $ 481.3 |  |  |  |  |
Accrued interest on surplus notes |  | 162.7 | 143.7 |  |  |  |  |
Supplemental Trust [Member] | BAM |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Special cash payment |  | 23.7 | 17.7 | $ 4.0 |  |  |  |
Accrued interest on surplus notes |  | 0.3 | 0.7 | 0.1 |  |  |  |
Surplus Note |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Assets Held-in-trust |  | 457.6 | 481.3 |  |  |  |  |
Surplus Note | BAM |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Special cash payment |  | 32.0 | 23.0 | 5.0 |  |  |  |
Accrued interest on surplus notes |  | $ 8.0 | $ 4.6 | 0.9 |  |  |  |
BAM | Fidus Re Ltd. |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Losses above retention, percentage of liability | 90.00% |  |  |  |  |  |  |
Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Insurance linked securities | $ 100.0 |  |  |  |  |  |  |
Collateralized financial guarantee, term | 12 years |  |  |  |  |  |  |
Collateral Financial Guarantee, Callable Term |  | 5 years |  |  |  |  |  |
Liabilities for guarantees on long-duration contracts, guaranteed benefit liability, gross | $ 165.0 |  |  |  |  |  |  |
Financial guarantee insurance contracts, reimbusement | $ 100.0 |  |  |  |  |  |  |
Percentage of Quota Share Reinsurance Agreement |  |  | 100.00% |  |  |  |  |
Direct written premiums |  |  | $ 2,200.0 |  |  |  |  |
Maximum | Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Liabilities for guarantees on long-duration contracts, guaranteed benefit liability, gross |  | $ 276.1 |  |  |  |  |  |
Series B BAM Surplus Note | HG Global |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  | $ 300.0
Series A BAM Surplus Note | HG Global |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Surplus notes |  |  |  | $ 203.0 |  |  |  |
Surplus Note | BAM |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Special cash payment |  | 32.0 | 23.0 |  |  |  |  |
Accrued interest on surplus notes |  | $ 8.0 | $ 4.6 |  |  |  |  |
Fixed interest rate | Surplus Note | BAM |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Interest rate (as a percent) |  | 8.00% |  |  |  |  |  |
Forecast | Surplus Note | BAM |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  |  |  | 4.60% |  |

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- Definition Municipal bond contracts issued and outstanding. + References No definition available. + Details
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- Definition Municipal bond outstanding interest + References No definition available. + Details
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Namespace Prefix: | wtm_
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Period Type: | instant

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- Definition Municipal bond outstanding principal + References No definition available. + Details
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Namespace Prefix: | wtm_
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- Definition Municipal bond debt service outstanding + References No definition available. + Details
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- Definition Percentage of par value of policy reinsured. + References No definition available. + Details
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- Definition Represents the percentage ceded under quota share reinsurance agreement. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
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- Definition Percentage of Risk Premium Ceded + References No definition available. + Details
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Namespace Prefix: | wtm_
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Balance Type: | na
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- Definition Percent over a target trust balance + References No definition available. + Details
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Namespace Prefix: | wtm_
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Balance Type: | na
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- Definition Supplement Trust Target + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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