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Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Net realized investment gains (losses), pre-tax | $ 25.7 | $ (5.1)
Net unrealized investment gains (losses), pre-tax (1) | 219.9 | (48.6)
Net realized and unrealized investment gains (losses), pre-tax | 245.6 | (53.7)
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses) | (52.2) | 5.7
Net realized and unrealized investment gains (losses), after-tax | 193.4 | $ (48.0)
MediaAlpha |  |
Unrealized investment gain | $ 114.7 |

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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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Period Type: | duration

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