XML 137 R52.htm IDEA: XBRL DOCUMENT
v3.19.1

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - Private equity security $ in Millions | Mar. 31, 2019 USD ($)
--- | ---
Debt and Equity Securities, FV-NI [Line Items] |
Fair Value | $ 97.6
1-3 years |
Debt and Equity Securities, FV-NI [Line Items] |
Fair Value | 7.0
3-5 years |
Debt and Equity Securities, FV-NI [Line Items] |
Fair Value | 0.0
5-10 years |
Debt and Equity Securities, FV-NI [Line Items] |
Fair Value | 67.9
Greater than 10 years |
Debt and Equity Securities, FV-NI [Line Items] |
Fair Value | $ 22.7

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Trading Security, Fair Value + References No definition available. + Details
Name: | wtm_TradingSecurityFairValue
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_PrivateEquityFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_RemainingLockupPeriodAxis=wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLiquidatingInvestmentRemainingPeriod1To3YearsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_RemainingLockupPeriodAxis=wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLiquidatingInvestmentRemainingPeriod3To5YearsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_RemainingLockupPeriodAxis=wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLiquidatingInvestmentRemainingPeriod5To10YearsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_RemainingLockupPeriodAxis=wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLiquidatingInvestmentRemainingPeriodOver10YearsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |