XML 83 R46.htm IDEA: XBRL DOCUMENT
v3.19.2

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2019 | Dec. 31, 2018
U.S. Government and agency obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | $ 183.5 | $ 154.0
Trading Securities, Unrealized Holding Gain | 1.4 | 0.1
Trading Securities, Unrealized Holding Loss | (0.1) | (0.9)
Fixed maturity investments, at fair value | 184.8 | 153.2
Debt securities issued by corporations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 446.1 | 519.0
Trading Securities, Unrealized Holding Gain | 11.7 | 1.0
Trading Securities, Unrealized Holding Loss | 0.0 | (9.5)
Fixed maturity investments, at fair value | 457.8 | 510.5
Municipal obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 286.3 | 279.0
Trading Securities, Unrealized Holding Gain | 11.2 | 2.4
Trading Securities, Unrealized Holding Loss | (0.1) | (1.1)
Fixed maturity investments, at fair value | 297.4 | 280.3
Mortgage-backed and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 168.2 | 136.1
Trading Securities, Unrealized Holding Gain | 1.7 | 0.1
Trading Securities, Unrealized Holding Loss | (0.5) | (2.7)
Fixed maturity investments, at fair value | 169.4 | 133.5
Fixed Income Investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 1,084.1 | 1,088.1
Trading Securities, Unrealized Holding Gain | 26.0 | 3.6
Trading Securities, Unrealized Holding Loss | (0.7) | (14.2)
Total fixed maturity investments including assets held for sale | $ 1,109.4 | $ 1,077.5

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 Represents the fair value of trading debt securities including assets classified as held for sale. + References No definition available. + Details
Name: | wtm_TradingSecuritiesDebtIncludingAssetsHeldForSale
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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

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

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

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