As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

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- Definition

Equity-based compensation cost during the period with respect to the award, which will be recognized in income (as well as the total recognized tax benefit) or capitalized as part of the cost of an asset.

+ References

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+ Details

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Period Type:	duration

- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

+ References

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(2)(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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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- Definition

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

+ Details

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

An excess of the fair value of the modified award over the fair value of the award immediately before the modification.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The number of shares newly issued during the reporting period under the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph 65
-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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-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
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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.

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-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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+ Details

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The total reduction to unamortized compensation cost for restricted shares forfeited during the period.

+ Details

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- Definition

The total fair value (on the date of issue) for restricted share awards issued during the period.

+ Details

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- Definition

Represents the number of installments in which the shares vest.

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

This element represents the percentage of restricted shares that would have vested for an individual terminated without cause between December 31, 2009 and December 31, 2010.

+ Details

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Namespace Prefix:	wtm
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Period Type:	duration

- Details

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Employee Share-Based Incentive Compensation Plans (Details 3) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended			6 Months Ended		12 Months Ended	3 Months Ended
	Mar. 31, 2007 Chairman and CEO WTM Restricted Stock	Jun. 30, 2011 WTM Restricted Stock	Jun. 30, 2010 WTM Restricted Stock	Jun. 30, 2011 WTM Restricted Stock	Jun. 30, 2010 WTM Restricted Stock	Dec. 31, 2010 WTM Restricted Stock	Mar. 31, 2010 Restricted shares that will vest in December, 2012
Activity in plan
Beginning of period (in shares)		73,250	98,750	46,250	91,900	91,900
Issued (in shares)	15,000	250		27,250	19,750		19,750
Vested (in shares)					(12,650)
Cancellations and change in assumed forfeitures (in shares)					(250)
Ending of period (in shares)		73,500	98,750	73,500	98,750	46,250
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance		$ 21.7	$ 27.6	$ 14.1	$ 23.7	$ 23.7
Issued		0.1		9.9	6.7
Forfeited					(0.1)
Modified			(3.3)		(3.3)
Expense recognized		(2.9)	(3.4)	(5.1)	(6.1)
Unamortized grant date fair value, ending balance		$ 18.9	$ 20.9	$ 18.9	$ 20.9	$ 14.1
Number of installments in which the shares vest						3
Issued (in shares)	15,000	250		27,250	19,750		19,750
Percentage of restricted shares that would have vested for a termination without cause (as a percent)						50.00%