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Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) (USD $) |  |  | 12 Months Ended |  | 12 Months Ended |  |  |  |  |  |  |  |  |  |  | 12 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  | 12 Months Ended |  |  | 12 Months Ended |  |  | 12 Months Ended |  |  | 12 Months Ended |  |  |  | 12 Months Ended |  |  |  |  |  |  |  |  |  |
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2012 Fair value measured on a recurring basis Discontinued Operations | Dec. 31, 2011 Fixed maturity investments | Dec. 31, 2012 Level 1 Inputs | Dec. 31, 2011 Level 1 Inputs | Dec. 31, 2012 Level 1 Inputs Fair value measured on a recurring basis | Dec. 31, 2011 Level 1 Inputs Fair value measured on a recurring basis | Dec. 31, 2012 Level 1 Inputs Fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2011 Level 1 Inputs Fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2012 Level 1 Inputs Common equity securities Fair value measured on a recurring basis | Dec. 31, 2011 Level 1 Inputs Common equity securities Fair value measured on a recurring basis | Dec. 31, 2012 Level 1 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2011 Level 1 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2012 Level 1 Inputs Other long-term investments Fair value measured on a recurring basis | Dec. 31, 2011 Level 1 Inputs Other long-term investments Fair value measured on a recurring basis | Dec. 31, 2012 Level 2 Inputs | Dec. 31, 2011 Level 2 Inputs | Dec. 31, 2012 Level 2 Inputs Fair value measured on a recurring basis | Dec. 31, 2011 Level 2 Inputs Fair value measured on a recurring basis | Dec. 31, 2012 Level 2 Inputs Fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2011 Level 2 Inputs Fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2012 Level 2 Inputs Common equity securities Fair value measured on a recurring basis | Dec. 31, 2011 Level 2 Inputs Common equity securities Fair value measured on a recurring basis | Dec. 31, 2012 Level 2 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2011 Level 2 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2012 Level 2 Inputs Other long-term investments Fair value measured on a recurring basis | Dec. 31, 2011 Level 2 Inputs Other long-term investments Fair value measured on a recurring basis | Dec. 31, 2012 Level 3 Inputs Fair value measured on a recurring basis | Dec. 31, 2011 Level 3 Inputs Fair value measured on a recurring basis | Dec. 31, 2012 Level 3 Inputs Fixed maturity investments Investment | Dec. 31, 2011 Level 3 Inputs Fixed maturity investments Investment | Dec. 31, 2012 Level 3 Inputs Fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2011 Level 3 Inputs Fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2012 Level 3 Inputs Common equity securities | Dec. 31, 2011 Level 3 Inputs Common equity securities | Dec. 31, 2012 Level 3 Inputs Common equity securities Fair value measured on a recurring basis | Dec. 31, 2011 Level 3 Inputs Common equity securities Fair value measured on a recurring basis | Dec. 31, 2012 Level 3 Inputs Convertible fixed maturity investments | Dec. 31, 2011 Level 3 Inputs Convertible fixed maturity investments | Dec. 31, 2012 Level 3 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2011 Level 3 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2012 Level 3 Inputs Other long-term investments | Dec. 31, 2011 Level 3 Inputs Other long-term investments | Dec. 31, 2012 Level 3 Inputs Other long-term investments Fair value measured on a recurring basis | Dec. 31, 2011 Level 3 Inputs Other long-term investments Fair value measured on a recurring basis | Dec. 31, 2010 Level 3 Inputs Other long-term investments Fair value measured on a recurring basis | Dec. 31, 2012 Fair value | Dec. 31, 2011 Fair value | Dec. 31, 2012 Fair value Fair value measured on a recurring basis | Dec. 31, 2011 Fair value Fair value measured on a recurring basis | Dec. 31, 2012 Fair value Fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2011 Fair value Fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2012 Fair value Common equity securities Fair value measured on a recurring basis | Dec. 31, 2011 Fair value Common equity securities Fair value measured on a recurring basis | Dec. 31, 2012 Fair value Convertible fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2011 Fair value Convertible fixed maturity investments Fair value measured on a recurring basis | Dec. 31, 2012 Fair value Other long-term investments Fair value measured on a recurring basis | Dec. 31, 2011 Fair value Other long-term investments Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value investments |  |  |  |  |  |  | $ 1,879,100,000 | $ 1,894,400,000 | $ 1,985,700,000 | $ 1,879,100,000 | $ 422,200,000 | [1] | $ 365,700,000 | [2] | $ 932,900,000 | $ 667,400,000 | $ 0 | $ 0 | $ 0 | [3] | $ 0 | [4] | $ 6,088,200,000 | $ 5,477,400,000 | $ 5,206,100,000 | $ 6,088,200,000 | $ 5,019,200,000 | [1] | $ 5,889,100,000 | [2] | $ 59,500,000 | $ 55,300,000 | $ 127,400,000 | $ 143,800,000 | $ 0 | [3] | $ 0 | [4] | $ 389,500,000 | $ 379,500,000 | $ 78,900,000 | $ 128,400,000 | $ 92,900,000 | [1] | $ 78,900,000 | [2] | $ 32,300,000 | $ 71,200,000 | $ 37,300,000 | $ 32,300,000 | $ 0 | $ 0.0 | $ 0 | $ 0 | $ 268,300,000 | [5] | $ 330,200,000 | [5] | $ 259,300,000 | [3] | $ 268,300,000 | [4] |  | $ 8,346,800,000 | [5],[6],[7],[8] | $ 7,901,600,000 | [5] | $ 7,581,300,000 | $ 8,346,800,000 | $ 5,534,300,000 | [1] | $ 6,333,700,000 | [2] | $ 1,029,700,000 | $ 755,000,000 | $ 127,400,000 | $ 143,800,000 | $ 259,300,000 | [3] | $ 268,300,000 | [4]
Total realized and unrealized gains (losses) |  |  |  |  |  |  | 46,800,000 | (1,400,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 53,600,000 | 113,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,700,000 | (8,100,000) |  |  |  |  | 12,400,000 | (4,700,000) |  |  | 0 | 0 |  |  | (3,300,000) |  | 19,500,000 |  |  |  |  |  |  | 118,200,000 |  | 118,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Foreign currency gains (losses) through OCI |  |  |  |  |  |  | 8,900,000 | 4,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 81,900,000 | (76,100,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 800,000 | (4,400,000) |  |  |  |  | 200,000 | 1,600,000 |  |  | 0 | 0 |  |  | 3,700,000 |  | (5,000,000) |  |  |  |  |  |  | 95,500,000 |  | (79,300,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization/Accretion |  |  |  |  |  |  | (800,000) | 2,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (48,000,000) | (54,200,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (800,000) | 300,000 |  |  |  |  | 0 | 0 |  |  | 0 | 0 |  |  | 0 |  | 0 |  |  |  |  |  |  | (49,600,000) |  | (51,600,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases |  |  |  |  |  |  | 7,266,500,000 | 10,653,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4,927,100,000 | 8,905,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 144,400,000 | 213,700,000 |  |  |  |  | 3,100,000 | 19,700,000 |  |  | 0 | 0 |  |  | 39,400,000 |  | 58,400,000 |  |  |  |  |  |  | 12,380,500,000 |  | 19,851,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  |  | (7,214,800,000) | (10,674,400,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (5,937,000,000) | (8,528,700,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (99,400,000) |  |  |  |  |  | (10,100,000) | (55,500,000) |  |  | 0 | 0 |  |  | (48,800,000) |  | (134,800,000) |  |  |  |  |  |  | (13,310,100,000) |  | (19,393,400,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Transfers in |  |  |  |  |  |  | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 62,400,000 | 269,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 22,100,000 | 18,200,000 |  |  |  |  | 0 | 0 |  |  | 0 | 0 |  |  | 0 |  | 0 |  |  |  |  |  |  | 84,500,000 |  | 287,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Transfers out |  |  |  |  |  |  | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (22,100,000) | (18,200,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (61,800,000) | (269,200,000) |  |  |  |  | (600,000) | 0 |  |  | 0 | 0 |  |  | 0 |  | 0 |  |  |  |  |  |  | (84,500,000) |  | (287,400,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value investments |  |  |  |  |  |  | 1,985,700,000 | 1,879,100,000 | 1,985,700,000 | 1,879,100,000 | 422,200,000 | [1] | 365,700,000 | [2] | 932,900,000 | 667,400,000 | 0 | 0 | 0 | [3] | 0 | [4] | 5,206,100,000 | 6,088,200,000 | 5,206,100,000 | 6,088,200,000 | 5,019,200,000 | [1] | 5,889,100,000 | [2] | 59,500,000 | 55,300,000 | 127,400,000 | 143,800,000 | 0 | [3] | 0 | [4] | 389,500,000 | 379,500,000 | 92,900,000 | 78,900,000 | 92,900,000 | [1] | 78,900,000 | [2] | 37,300,000 | 32,300,000 | 37,300,000 | 32,300,000 | 0 | 0 | 0 | 0 | 259,300,000 |  | 268,300,000 | [5] | 259,300,000 | [3] | 268,300,000 | [4] |  | 7,581,300,000 | [7],[8] | 8,346,800,000 | [5],[6],[7],[8] | 7,581,300,000 | 8,346,800,000 | 5,534,300,000 | [1] | 6,333,700,000 | [2] | 1,029,700,000 | 755,000,000 | 127,400,000 | 143,800,000 | 259,300,000 | [3] | 268,300,000 | [4]
Carrying value of investment accounted for using the equity method |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 35,000,000 |  | 33,000,000 |  | 41,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Change in fair value of derivatives, included in other revenues |  |  |  |  | (100,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed maturity investments reclassified to assets held for sale (1) | (338,100,000) | [9] | (111,800,000) | [9] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 | 10 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Securities of level 3 measurements recategorized as level 2 measurements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 61,800,000 | 269,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 22,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Difference in value between securities determined using the industry standard pricing models and securities based upon quoted prices provided by a third party |  |  |  |  |  | $ 1,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Carrying value includes $338.1 that is classified as assets held for sale relating to discontinued operations.
[2] | Carrying value includes $111.8 that is classified as assets held for sale relating to discontinued operations.
[3] | Excludes carrying value of $35.0 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to currency forward contracts.
[4] | Excludes carrying value of $33.0 associated with other long-term investments accounted for using the equity method.
[5] | Excludes carrying value of $33.0 and $41.9 at December 31, 2011 and January 1, 2011 associated with other long-term investment limited partnerships accounted for using the equity method.
[6] | Carrying value includes $111.8 that is classified as assets held for sale relating to AutoOne discontinued operations.
[7] | Excludes carrying value of $33.0 and $35.0 at January 1, 2012 and December 31, 2012 associated with other long-term investments accounted for using the equity method and $(0.1) at December 31, 2012 related to currency forward contracts.
[8] | Carrying value includes $111.8 and $338.1 at January 1, 2012 and December 31, 2012 that is classified as assets held for sale relating to discontinued operations.
[9] | Assets held for sale related to discontinued operations. See Note 20.

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- Definition The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Net Increase or Decrease in the fair value of the derivative or group of derivatives included in earnings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959 + Details
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- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents investments accounted for under the equity method of accounting as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 18 -Paragraph 6 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6903542&loc=d3e32014-111567 + Details
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- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents investment securities as of the balance sheet date which may include marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 13 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 12 -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=16383099&loc=d3e24584-111560 + Details
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- Definition This element represents the amount of the total realized and unrealized (holding) gains or losses for the period which are included in the statement of income. + References No definition available. + Details
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- Definition Represents the difference between the estimated fair value for security determined using the industry standard pricing models and the estimated fair value based upon quoted prices provided by a third party vendor. + References No definition available. + Details
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- Definition Represents the securities recategorized from one fair value measurement level to another because quoted market prices for similar securities that were considered reliable could be validated against an alternative source that was available at the period end date. + References No definition available. + Details
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Period Type: | instant

X
- Definition Represents the fair value of trading debt securities re-classified to assets held for sale. + References No definition available. + Details
Name: | wtm_TradingSecuritiesDebtReclassifiedToAssetsHeldForSale
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant