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Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Net realized investment gains (losses), pre-tax | $ 45.5 | $ 1.4 | $ 71.2 | $ (3.7)
Net unrealized investment gains (losses), pre-tax | (10.3) | 9.1 | 209.6 | (39.5)
Net realized and unrealized investment gains (losses), pre-tax | 35.2 | 10.5 | 280.8 | (43.2)
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses) | (4.1) | (1.9) | (56.3) | 3.8
Net realized and unrealized investment gains (losses), after-tax | $ 31.1 | $ 8.6 | 224.5 | (39.4)
Net gain from sale of discontinued operations, net of tax |  |  | 67.5 | $ 0.0
MediaAlpha |  |  |  |
Unrealized investment gain |  |  | 114.7 |
Net gain from sale of discontinued operations, net of tax |  |  | $ 67.5 |

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- Definition The sum of the realized and unrealized net gain (loss) on trading securities during the period. + References No definition available. + Details
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- Definition Represents the equity in unrealized gains or losses from investments in unconsolidated affiliates. + References No definition available. + Details
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Namespace Prefix: | wtm_
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- Definition Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax + References No definition available. + Details
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- 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
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- 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

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- Definition Tax on net realized and unrealized gains (loss), trading securities + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition Trading Securities, Realized Gain (Loss), Total + References No definition available. + Details
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- Details
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