XML 62 R42.htm IDEA: XBRL DOCUMENT
v3.19.3

Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Net realized investment gains (losses), pre-tax (1) | $ 19,600,000 |  | $ (10,200,000) | $ 90,800,000 | $ (13,900,000)
Net unrealized investment gains (losses), pre-tax | 47,400,000 |  | 76,300,000 | 257,000,000.0 | 36,800,000
Net realized and unrealized investment gains, pre-tax | 67,000,000.0 |  | 66,100,000 | 347,800,000 | 22,900,000
Income tax expense attributable to net realized and unrealized investment gains | (13,200,000) |  | (9,700,000) | (69,500,000) | (5,900,000)
Net realized and unrealized investment gains, after-tax | 53,800,000 |  | $ 56,400,000 | 278,300,000 | 17,000,000.0
Net gain from sale of discontinued operations, net of tax |  |  |  | 67,500,000 | $ 0
MediaAlpha |  |  |  |  |
Net gain from sale of discontinued operations, net of tax |  |  |  | 67,500,000 |
Unrealized investment gain | $ 35,300,000 | $ 114,700,000 |  | $ 150,000,000.0 |

X
- Definition The sum of the realized and unrealized net gain (loss) on trading securities during the period. + References No definition available. + Details
Name: | us-gaap_NetRealizedOrUnrealizedGainLossOnTradingSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Represents the equity in unrealized gains or losses from investments in unconsolidated affiliates. + References No definition available. + Details
Name: | wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax + References No definition available. + Details
Name: | wtm_NetRealizedandUnrealizedGainLossonTradingSecuritiesNetofTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. + References No definition available. + Details
Name: | wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Realized Gain From Transaction + References No definition available. + Details
Name: | wtm_RealizedGainFromTransaction
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Tax on net realized and unrealized gains (loss), trading securities + References No definition available. + Details
Name: | wtm_Taxonnetrealizedandunrealizedgainslosstradingsecurities
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Trading Securities, Realized Gain (Loss), Total + References No definition available. + Details
Name: | wtm_TradingSecuritiesRealizedGainLossTotal
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=wtm_MediaAlphaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |