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

The sum of the realized and unrealized net gain (loss) on trading securities during the period.

No definition available.

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Represents the equity in unrealized gains or losses from investments in unconsolidated affiliates.

No definition available.

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Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax

No definition available.

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The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period.

No definition available.

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Realized Gain From Transaction

No definition available.

Name:	wtm_RealizedGainFromTransaction
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Tax on net realized and unrealized gains (loss), trading securities

No definition available.

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Data Type:	xbrli:monetaryItemType
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Trading Securities, Realized Gain (Loss), Total

No definition available.

Name:	wtm_TradingSecuritiesRealizedGainLossTotal
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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Period Type: