Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Goodwill and Intangible Assets

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Name:	wtm_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedGoodwillandIntangibleAssets
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Millions	May 18, 2018	May 11, 2018
NSM Insurance HoldCo, LLC
Goodwill [Line Items]
Goodwill and intangible assets acquired		$ 381.9
Fresh Insurance Services Group Limited
Goodwill [Line Items]
Goodwill and intangible assets acquired	$ 54.3