XML 140 R120.htm IDEA: XBRL DOCUMENT v3.6.0.2

Income Taxes (ASC 740-10) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Percentage of likelihood for realization of benefit upon ultimate settlement	50.00%
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance	$ 24.3	$ 35.0	$ 33.7
Changes in prior year tax positions	(5.4)	(10.7)	(4.9)
Lapse in statute of limitations			(1.1)
Settlements with tax authorities	(13.7)
Tax positions taken during the current year			7.3
Unrecognized tax benefit recognition including interest and penalties, if recognized, would effect income tax expense	5.2
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	(8.2)	1.7	(2.2)
Ending balance	5.2	24.3	35.0
Permanent Differences [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance	5.4	5.4	8.4
Changes in prior year tax positions	0.0	0.0	(2.2)
Lapse in statute of limitations			(0.8)
Settlements with tax authorities	(5.4)
Tax positions taken during the current year			0.0
Ending balance	0.0	5.4	5.4
Temporary Differences [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance	10.7	23.1	16.6
Changes in prior year tax positions	(5.5)	(12.4)	(0.8)
Lapse in statute of limitations			0.0
Settlements with tax authorities	0.0
Tax positions taken during the current year			7.3
Ending balance	5.2	10.7	23.1
Interest and Penalties Net of Tax Benefit [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits, Interest on Income Taxes Accrued		8.2
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns
Beginning balance		6.5	8.7
Changes in prior year tax positions	0.1	$ 1.7	(1.9)
Lapse in statute of limitations			(0.3)
Settlements with tax authorities	(8.3)
Tax positions taken during the current year			0.0
Ending balance	$ 0.0		$ 6.5

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

X

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of unrecognized tax benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Unrecognized Tax Benefit
-URI http://asc.fasb.org/extlink&oid=6527854

+ Details

Name:	us-gaap_UnrecognizedTaxBenefits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of decrease in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of expense for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32718-109319

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319

+ Details

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

X

- Definition

Amount of interest expense accrued for an underpayment of income taxes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32718-109319

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Represents percentage of likelihood for realization of benefit upon ultimate settlement if the recognition threshold is met.

+ References

No definition available.

+ Details

Name:	wtm_PercentageOfLikelihoodForRealizationOfBenefitUponUltimateSettlement
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis=wtm_PermanentDifferencesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis=wtm_TemporaryDifferencesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis=wtm_InterestAndPenaltiesNetOfTaxBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: