XML 84 R47.htm IDEA: XBRL DOCUMENT
v3.19.2

Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2019 | Dec. 31, 2018
Common Stock |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | $ 616.6 | $ 904.7
Trading Securities, Unrealized Holding Gain | 92.6 | 51.0
Trading Securities, Unrealized Holding Loss | 0.0 | (30.1)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 709.2 | 925.6
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 561.9 | 330.3
Trading Securities, Unrealized Holding Gain | 168.2 | 52.2
Trading Securities, Unrealized Holding Loss | (63.7) | (54.9)
Net foreign currency gains (losses) | (2.0) | (2.0)
Fixed maturity investments, at fair value | $ 664.4 | $ 325.6

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 Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading. + References No definition available. + Details
Name: | us-gaap_TradingSecuritiesCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 + Details
Name: | us-gaap_TradingSecuritiesDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of unrealized holding gain before loss recognized in the income statement for investments in trading securities. + References No definition available. + Details
Name: | us-gaap_TradingSecuritiesUnrealizedHoldingGain
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of unrealized holding loss before gain recognized in the income statement for investments in trading securities. + References No definition available. + Details
Name: | us-gaap_TradingSecuritiesUnrealizedHoldingLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The net amount of unrealized gains (losses) from foreign currency transaction adjustments related to trading securities. + References No definition available. + Details
Name: | wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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