Investment Securities (Details 9) (USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2011	Dec. 31, 2010
Fixed maturity investments \| Level 3 Inputs
Investment securities
Fair value investments	$ 216.9	$ 128.4
Total realized and unrealized gains (losses)	(10.6)
Foreign currency gains (losses) through OCI and other revenue	(4.4)
Amortization/Accretion	(0.1)
Purchases	212.8
Sales	1.6
Transfers in	1.0
Transfers out	(111.8)
Common Equity Securities \| Level 3 Inputs
Investment securities
Fair value investments	32.1	71.2
Total realized and unrealized gains (losses)	(4.9)
Foreign currency gains (losses) through OCI and other revenue	1.6
Purchases	19.7
Sales	(55.5)
Other long-term investments. \| Level 3 Inputs
Investment securities
Fair value investments	292.2	330.2
Total realized and unrealized gains (losses)	22.5
Foreign currency gains (losses) through OCI and other revenue	(5.0)
Purchases	27.9
Sales	(83.4)
Carrying value of investment accounted for using the equity method	36.6	41.9
Level 1 Inputs
Investment securities
Fair value investments	1,755.2	1,894.4
Total realized and unrealized gains (losses)	(50.5)
Foreign currency gains (losses) through OCI and other revenue	0.3
Amortization/Accretion	2.4
Purchases	6,690.7
Sales	(6,782.1)
Level 2 Inputs
Investment securities
Fair value investments	4,969.5	5,477.4
Total realized and unrealized gains (losses)	115.9
Foreign currency gains (losses) through OCI and other revenue	(84.4)
Amortization/Accretion	(40.7)
Purchases	3,987.4
Sales	(4,596.9)
Transfers in	111.8
Transfers out	(1.0)
Securities of level 3 measurements recategorized as level 2 measurements	111.8
Number of securities with unobservable inputs	1
Level 3 Inputs
Investment securities
Number of securities with unobservable inputs	7
Difference in value between securities determined using the industry standard pricing models and securities based upon quoted prices provided by a third party	0.8
Fair Value
Investment securities
Fair value investments	7,265.9	7,901.6
Total realized and unrealized gains (losses)	72.4
Foreign currency gains (losses) through OCI and other revenue	(91.9)
Amortization/Accretion	(38.4)
Purchases	10,938.5
Sales	(11,516.3)
Transfers in	112.8
Transfers out	$ (112.8)

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.

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-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.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Section 45

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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.

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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 Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Name Statement of Financial Accounting Standard (FAS)

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 1

-Subparagraph a, f, g

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Adjustment that results from the process of translating subsidiary financial statements and foreign equity investments into the reporting currency of the reporting entity, net of tax.

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-Name Accounting Standards Codification

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-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 AICPA

-Name Accounting Research Bulletin (ARB)

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-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 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-Paragraph 14, 17, 19, 26

-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 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 52

-Paragraph 13, 20, 31

-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.

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- Definition

The cash outflow associated with the purchase of all investments (debt, security, other) during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 15, 17

-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

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

The cash inflow associated with the sale, maturities, repayments and calls of securities classified as trading securities and held for investment purposes. Excludes proceeds from trading securities purchased and held principally for the purpose of selling them in the near term (thus held for only a short period of time).

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- 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.

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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.

+ Details

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This element represents transfers in to one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis.

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This element represents transfers out of one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis.

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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.

+ Details

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Namespace Prefix:	wtm
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Period Type:	instant

- Definition

Represents the number of securities with unobservable inputs during the current period.

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Balance Type:	na
Period Type:	duration

- 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.

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