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Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Earnings Per Share [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net income (loss) attributable to White Mountains’ common shareholders |  |  |  |  |  |  |  |  | $ 316.1 us-gaap_IncomeLossFromContinuingOperations | $ 317.3 us-gaap_IncomeLossFromContinuingOperations | $ 322.4 us-gaap_IncomeLossFromContinuingOperations
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |  |  |  |  |  |  |
Allocation of income for unvested restricted common shares(1) |  |  |  |  |  |  |  |  | (4.1)wtm_IncomeLossAllocatedToParticipatingUnvestedRestrictedCommonShares | (4.7)wtm_IncomeLossAllocatedToParticipatingUnvestedRestrictedCommonShares | (4.3)wtm_IncomeLossAllocatedToParticipatingUnvestedRestrictedCommonShares
Dividends declared on participating restricted common shares(1) |  |  |  |  |  |  |  |  | (0.1)us-gaap_DividendsShareBasedCompensation | (0.1)us-gaap_DividendsShareBasedCompensation | (0.1)us-gaap_DividendsShareBasedCompensation
Total allocation to restricted common shares |  |  |  |  |  |  |  |  | (4.2)wtm_AggregateAllocationToRestrictedSharesIncludingDividend | (4.8)wtm_AggregateAllocationToRestrictedSharesIncludingDividend | (4.4)wtm_AggregateAllocationToRestrictedSharesIncludingDividend
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts |  |  |  |  |  |  |  |  | 311.9 wtm_IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet | 312.5 wtm_IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet | 318.0 wtm_IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet
Undistributed net earnings (in millions): |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts |  |  |  |  |  |  |  |  | 311.9 wtm_IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet | 312.5 wtm_IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet | 318.0 wtm_IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet
Dividends declared net of restricted common share amounts(1) |  |  |  |  |  |  |  |  | (6.1)wtm_DividendsCommonStockCashNetOfRestrictedCommonShareAmounts | (6.1)wtm_DividendsCommonStockCashNetOfRestrictedCommonShareAmounts | (6.5)wtm_DividendsCommonStockCashNetOfRestrictedCommonShareAmounts
Total undistributed net earnings, net of restricted common share amounts |  |  |  |  |  |  |  |  | $ 305.8 us-gaap_UndistributedEarnings | $ 306.4 us-gaap_UndistributedEarnings | $ 311.5 us-gaap_UndistributedEarnings
Basic earnings per share denominators (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 6,104,900 us-gaap_WeightedAverageNumberOfSharesIssuedBasic | 6,200,400 us-gaap_WeightedAverageNumberOfSharesIssuedBasic | 6,799,800 us-gaap_WeightedAverageNumberOfSharesIssuedBasic
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (78,900)us-gaap_WeightedAverageNumberOfSharesRestrictedStock | (91,400)us-gaap_WeightedAverageNumberOfSharesRestrictedStock | (91,100)us-gaap_WeightedAverageNumberOfSharesRestrictedStock
Basic earnings per share denominator |  |  |  |  |  |  |  |  | 6,026,000 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 6,109,000 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 6,708,700 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Diluted earnings per share denominator (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 6,104,900 us-gaap_WeightedAverageNumberOfSharesIssuedBasic | 6,200,400 us-gaap_WeightedAverageNumberOfSharesIssuedBasic | 6,799,800 us-gaap_WeightedAverageNumberOfSharesIssuedBasic
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (78,900)us-gaap_WeightedAverageNumberOfSharesRestrictedStock | (91,400)us-gaap_WeightedAverageNumberOfSharesRestrictedStock | (91,100)us-gaap_WeightedAverageNumberOfSharesRestrictedStock
Average outstanding dilutive options to acquire common shares |  |  |  |  |  |  |  |  | 0 us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment | 0 us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment | 0 us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
Diluted earnings (loss) per share denominator |  |  |  |  |  |  |  |  | 6,026,000 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding | 6,109,000 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding | 6,708,700 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Basic earnings per share (in dollars): |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains’s common shareholders | $ 13.71 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 7.35 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 15.10 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 15.56 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 19.14 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 9.20 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 3.62 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 19.10 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 51.77 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 51.15 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 47.41 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare
Dividends declared and paid |  |  |  |  |  |  |  |  | $ (1.00)us-gaap_CommonStockDividendsPerShareDeclared | $ (1.00)us-gaap_CommonStockDividendsPerShareDeclared | $ (1.00)us-gaap_CommonStockDividendsPerShareDeclared
Undistributed earnings |  |  |  |  |  |  |  |  | $ 50.77 us-gaap_EarningsPerShareBasicUndistributed | $ 50.15 us-gaap_EarningsPerShareBasicUndistributed | $ 46.41 us-gaap_EarningsPerShareBasicUndistributed
Diluted earnings per share (in dollars) |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains’s common shareholders | $ 13.71 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 7.35 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 15.10 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 15.56 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 19.14 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 9.20 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 3.62 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 19.10 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 51.77 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 51.15 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 47.41 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare
Dividends declared and paid |  |  |  |  |  |  |  |  | $ (1.00)us-gaap_CommonStockDividendsPerShareDeclared | $ (1.00)us-gaap_CommonStockDividendsPerShareDeclared | $ (1.00)us-gaap_CommonStockDividendsPerShareDeclared
Undistributed earnings |  |  |  |  |  |  |  |  | $ 50.77 us-gaap_EarningsPerShareDilutedUndistributed | $ 50.15 us-gaap_EarningsPerShareDilutedUndistributed | $ 46.41 us-gaap_EarningsPerShareDilutedUndistributed
Incremental common shares attributable to share-based payment arrangements |  |  |  |  |  |  |  |  | 125,000 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements |  |

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- Definition The earnings attributable to each security on the basis that all of the earnings for the period had been distributed (to the extent that each security may share in earnings). This is the earnings amount which reflects the participation rights in undistributed earnings of each security. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60B -URI http://asc.fasb.org/extlink&oid=7655603&loc=SL5780133-109256 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60A -URI http://asc.fasb.org/extlink&oid=7655603&loc=SL5780132-109256 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e2740-109256 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 66 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e2814-109256 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 65 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e2793-109256 + Details
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- Definition The earnings attributable to each security on the basis that all of the earnings for the period had been distributed (to the extent that each security may share in earnings). This is the earnings amount which reflects the participation rights in undistributed earnings of each security, after giving effect to all dilutive potential common shares outstanding during the period. + References No definition available. + Details
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- Definition Number of shares of restricted stock determined by relating the portion of time within a reporting period that restricted shares have been outstanding to the total time in that period. Restricted shares are subject to sales, contractual, regulatory or other restrictions that prevent or inhibit the holder from freely disposing of them before the restriction ends. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e2646-109256 + Details
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- Definition Represents Net income from continuing operations attributable to White Mountains common shareholders, net of allocation to restricted shares and dividends paid on restricted shares. + References No definition available. + Details
Name: | wtm_IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration