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Municipal Bond Guarantee - Narrative (Details) $ in Millions | 1 Months Ended | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Apr. 30, 2018 USD ($) | Jun. 30, 2019 USD ($) yr Contract | Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) Contract | Dec. 31, 2017 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2019 | Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Repayments of debt |  | $ 21.7 | $ 6.1 |  |  |  |  |
Interest paid |  | 7.3 | $ 0.7 |  |  |  |  |
Percentage of discounted future cash flows |  |  |  |  | 10.00% |  |  |
Total gross unearned insurance premiums |  | 187.8 |  | $ 176.0 |  |  |  |
HG Global |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Partners capital contribution |  |  |  |  |  | $ 594.5 |  |
Surplus notes |  | $ 481.3 |  |  |  |  |  | $ 503.0
Assets held-in-trust |  |  |  | $ 603.0 |  |  |  |
HG Global | Preferred Stock |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  | 96.90% |  | 96.90% |  |  |  |
HG Global | Common Stock |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  | 88.40% |  | 88.40% |  |  |  |
BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Percentage of risk premium ceded |  | 60.00% |  |  |  |  |  |
Year of U.S. Treasury rate | yr |  | 1 |  |  |  |  |  |
Contracts outstanding | Contract |  | 8,126 |  | 7,525 |  |  |  |
Remaining weighted average contract period outstanding (in years) |  | 10 years 9 months 18 days |  | 10 years 8 months 12 days |  |  |  |
Municipal bond outstanding principal |  | $ 56,043.9 |  | $ 52,201.6 |  |  |  |
Municipal bond outstanding interest |  | 28,591.7 |  | 26,560.3 |  |  |  |
Municipal bond debt service outstanding |  | 84,635.6 |  | 78,761.9 |  |  |  |
July 1, 2019 - December 31, 2019 |  | 8.5 |  |  |  |  |  |
2020 |  | 16.2 |  |  |  |  |  |
2021 |  | 15.3 |  |  |  |  |  |
2022 |  | 14.4 |  |  |  |  |  |
2023 |  | 13.6 |  |  |  |  |  |
2024 |  | 12.8 |  |  |  |  |  |
2025 and thereafter |  | 107.0 |  |  |  |  |  |
Total gross unearned insurance premiums |  | 187.8 |  |  |  |  |  |
HG Global |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Collateral held in supplement trust |  | 777.5 |  | 757.4 |  |  |  |
Interest receivable |  | $ 157.4 |  | 143.7 |  |  |  |
HG Re |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured |  | 15.00% |  |  |  |  |  |
BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Surplus notes |  | $ 481.3 |  | $ 481.3 |  |  |  |
BAM Management |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Partners capital contribution |  |  |  |  |  | $ 14.5 |  |
Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Derivative, fixed interest rate (as a percent) |  |  |  | 4.60% |  |  |  |
Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Repayments of debt |  |  |  | $ 17.7 |  |  |  |
Interest paid |  |  |  | $ 5.3 |  |  |  |
Combination One | Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Number of years of change in interest rate | yr |  | 5 |  |  |  |  |  |
US Treasury Rate | Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate |  | 3.00% |  |  |  |  |  |
Fixed interest rate | Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Derivative, fixed interest rate (as a percent) |  | 8.00% |  | 8.00% |  |  |  |
First Quarter, year two | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
2020 |  | $ 4.1 |  |  |  |  |  |
Second Quarter, year two | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
2020 |  | 4.1 |  |  |  |  |  |
Third Quarter, year two | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
2020 |  | 4.0 |  |  |  |  |  |
Fourth Quarter, year two | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
2020 |  | 4.0 |  |  |  |  |  |
Series A BAM Surplus Note | HG Global |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  | $ 203.0 |  |  |
Series B BAM Surplus Note | HG Global |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  | $ 300.0 |  |  |
BAM | Fidus Re Ltd. |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Losses above retention, percentage of liability | 90.00% |  |  |  |  |  |  |
Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Secured Debt | $ 100.0 |  |  |  |  |  |  |
Collateralized financial guarantee, term | 12 years |  |  |  |  |  |  |
Collateral financial guarantee, callable term |  |  |  | 5 years |  |  |  |
Aggregate losses on ceded business | $ 165.0 |  |  |  |  |  |  |
Premium receivable, Premium expected to be collected | 100.0 |  |  |  |  |  |  |
Percentage of Quota Share Reinsurance Agreement |  |  |  | 100.00% |  |  |  |
Direct written premiums |  |  |  | $ 2,200.0 |  |  |  |
Minimum | Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Aggregate losses on ceded business | $ 165.0 |  |  |  |  |  |  |
Maximum | Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Aggregate losses on ceded business |  | $ 276.1 |  |  |  |  |  |
Scenario, Forecast | Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Derivative, fixed interest rate (as a percent) |  |  |  |  |  |  | 5.70% |

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- Definition Represents the percentage ceded under quota share reinsurance agreement. + References No definition available. + Details
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- Definition Percentage of Risk Premium Ceded + References No definition available. + Details
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- Definition Percentage of discounted future cash flows + References No definition available. + Details
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Namespace Prefix: | wtm_
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Balance Type: | na
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- Definition Year of U.S. Treasury Rate + References No definition available. + Details
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Namespace Prefix: | wtm_
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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
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- Details
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Balance Type: |
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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
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- Details
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- Details
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- Details
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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
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- Details
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Balance Type: |
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- Details
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Namespace Prefix: |
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- Details
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- Details
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- Details
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- Details
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Namespace Prefix: |
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Balance Type: |
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
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- Details
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