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Subsequent Event (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Repayments of debt, principal		$ 22.9	$ 15.4	$ 365.0
Subsequent Event
Subsequent Event [Line Items]
Maximum exposure to any single issuer	$ 75.0
BAM | Subsequent Event
Subsequent Event [Line Items]
Accrued interest on surplus notes	146.5
Surplus notes	409.7
Surplus Note | BAM
Subsequent Event [Line Items]
Special cash payment		32.0	23.0
Repayments of debt, principal		23.7	17.7
Accrued interest held in Supplemental Trust		0.3	0.7
Accrued interest on surplus notes		$ 8.0	$ 4.6
Surplus Note | BAM | Subsequent Event
Subsequent Event [Line Items]
Special cash payment	65.0
Repayments of debt, principal	47.9
Accrued interest held in Supplemental Trust	0.9
Accrued interest on surplus notes	$ 16.2

X

- Definition

Amount of allowance for credit loss on reinsurance recoverable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118955202&loc=SL82895884-210446

+ Details

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Balance Type:	credit
Period Type:	instant

X

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585

+ Details

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Period Type:	duration

X

- Definition

The cash outflow to repay long-term debt that is not secured by collateral. Excludes repayments of tax exempt unsecured debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585

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X

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

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X

- Definition

Financial instruments issued by insurance entities (surplus notes) that are includable in surplus for statutory accounting purposes as prescribed or permitted by state laws and regulations that are liabilities of the issuer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6486086&loc=d3e24387-158525

+ Details

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X

- Definition

Accrued interest payment on surplus notes issued by VIE. The interest is eliminated in consolidation with the primary beneficiary.

+ References

No definition available.

+ Details

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Balance Type:	debit
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X

- Definition

Accrued Interest payment for Supplemental Trust

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
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Balance Type:	debit
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- Details

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- Details

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