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Investment Securities (Investments Held on Deposit or as Collateral) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Investments held in trusts	$ 319.9	$ 254.3
Assets held by insurance regulators	$ 9.9	$ 6.1
Percentage of investments recorded at fair value	71.00%	87.00%

X

- Definition

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=75763970&loc=SL75763979-209797

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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- Definition

Investments Held In Trusts

+ References

No definition available.

+ Details

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

X

- Definition

Represents the percentage of the total investment portfolio recorded at fair value based upon quoted market prices or other observable inputs.

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
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