XML 73 R55.htm IDEA: XBRL DOCUMENT v3.20.4

Significant Transactions - Ark (Details) - USD ($) $ in Millions	Oct. 01, 2020	Dec. 31, 2020	Dec. 31, 2019
Other Operations
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction		$ 646.3	$ 0.0
Ark Insurance Holdings Limited
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction		366.3
Ark Insurance Holdings Limited | Lloyd
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction		$ 280.0
Ark Insurance Holdings Limited
Other Significant Noncash Transactions [Line Items]
Merger agreement, equity capital contributed	$ 605.4
Equity capital, pre money valuation amount	300.0
Additional equity purchase from shareholders	40.9
Investment, potential additional contribution of equity capital	$ 200.0

X

- Definition

Amount of equity interests of the acquirer, including instruments or interests issued or issuable in consideration for the business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6613-128477

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6578-128477

+ Details

Name:	us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_EscrowDeposit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherSignificantNoncashTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Future Commitment

+ References

No definition available.

+ Details

Name:	wtm_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuableFutureCommitment
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Pre Money Valuation

+ References

No definition available.

+ Details

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

X

- Definition

Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Equity Acquired

+ References

No definition available.

+ Details

Name:	wtm_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquityAcquired
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	dei_LegalEntityAxis=wtm_OtherEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=wtm_ArkInsuranceHoldingsLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_CounterpartyNameAxis=wtm_LloydMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=wtm_ArkInsuranceHoldingsLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: