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Discontinued Operations - Earnings Per Share (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Net loss from discontinued operations, net of tax $ (0.1)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity $ (0.5)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity
Income (Loss) Allocated to Participating Unvested Restricted Common Shares (0.7)wtm_IncomeLossAllocatedToParticipatingUnvestedRestrictedCommonShares (1.1)wtm_IncomeLossAllocatedToParticipatingUnvestedRestrictedCommonShares
Net Income (Loss) Available to Common Stockholders, Basic 84.3us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic 95.5us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Total average common shares outstanding during the period 5,978,400us-gaap_WeightedAverageNumberOfSharesIssuedBasic 6,169,900us-gaap_WeightedAverageNumberOfSharesIssuedBasic
Weighted Average Number of Shares, Restricted Stock (58,800)us-gaap_WeightedAverageNumberOfSharesRestrictedStock (70,400)us-gaap_WeightedAverageNumberOfSharesRestrictedStock
Basic earnings per share denominator 5,919,600us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 6,099,500us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Average outstanding dilutive options to acquire common shares 0us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment 0us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
Diluted earnings (loss) per share denominator 5,919,600us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 6,099,500us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements 125,000us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 125,000us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements
Segment, Discontinued Operations    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Net loss from discontinued operations, net of tax 7.9us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
(0.5)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Income (Loss) Allocated to Participating Unvested Restricted Common Shares (0.1)wtm_IncomeLossAllocatedToParticipatingUnvestedRestrictedCommonShares
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
0wtm_IncomeLossAllocatedToParticipatingUnvestedRestrictedCommonShares
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Net Income (Loss) Available to Common Stockholders, Basic $ 7.8us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
$ (0.5)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Total average common shares outstanding during the period 5,978,400us-gaap_WeightedAverageNumberOfSharesIssuedBasic
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
6,169,900us-gaap_WeightedAverageNumberOfSharesIssuedBasic
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Weighted Average Number of Shares, Restricted Stock (58,800)us-gaap_WeightedAverageNumberOfSharesRestrictedStock
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
(70,400)us-gaap_WeightedAverageNumberOfSharesRestrictedStock
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Basic earnings per share denominator 5,919,600us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
6,099,500us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Average outstanding dilutive options to acquire common shares 0us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
0us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Diluted earnings (loss) per share denominator 5,919,600us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
6,099,500us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Earnings Per Share, Basic and Diluted $ 1.32us-gaap_EarningsPerShareBasicAndDiluted
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
$ (0.08)us-gaap_EarningsPerShareBasicAndDiluted
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements   125,000us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember