The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 6A

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=SL6742756-110258

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Distributions pending from hedge funds that are no longer active and are in process of disposing their underlying investments.

+ References

No definition available.

+ Details

Name:	wtm_DistributionsFromInactiveHedgeFunds
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of redemption requests submitted for investments for hedge funds.

+ References

No definition available.

+ Details

Name:	wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareRedemptionRequest
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the maximum price difference provided by the pricing services between the market price and the actual sale price of securities, on the basis of which outliers are considered.

+ References

No definition available.

+ Details

Name:	wtm_MaximumDifferencesBetweenPricesProvidedByPricingServicesConsideredOutliers
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the maximum percentage of price difference provided by the pricing services between the market price and the actual sale price of securities, on the basis of which outliers are considered.

+ References

No definition available.

+ Details

Name:	wtm_MaximumPercentageDifferencesBetweenPricesProvidedByPricingServicesConsideredOutliers
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of investments held.

+ References

No definition available.

+ Details

Name:	wtm_NumberOfInvestmentsHeld
Namespace Prefix:	wtm_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	wtm_OtherLongTermInvestmentsAbstract
Namespace Prefix:	wtm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the total fair market value of investments in certain entities that calculate net asset value per share (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements, but may also be used in both the balance sheet and disclosure in the same submission. This item represents alternative investments as of the balance sheet date which may include investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds, and any other investment in which investee management calculates net asset value per share as the primary metric for investment value.

+ References

No definition available.

+ Details

Name:	wtm_OtherLongTermInvestmentsFairValueDisclosure
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The largest investment which the entity has made in a single fund.

+ References

No definition available.

+ Details

Name:	wtm_OtherLongTermInvestmentsLargestSingleInvestment
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Investment Securities (Details 5) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2012 Hedge and private equity funds included in other long-term investments	Dec. 31, 2011 Hedge and private equity funds included in other long-term investments	Mar. 31, 2012 Hedge funds entity	Dec. 31, 2011 Hedge funds	Mar. 31, 2012 Hedge funds 30-59 days notice	Mar. 31, 2012 Hedge funds 60-89 days notice	Mar. 31, 2012 Hedge funds 90-119 days notice	Mar. 31, 2012 Hedge funds 120+ days notice	Mar. 31, 2012 Hedge funds Monthly	Mar. 31, 2012 Hedge funds Monthly 120+ days notice	Mar. 31, 2012 Hedge funds Quarterly	Mar. 31, 2012 Hedge funds Quarterly 30-59 days notice	Mar. 31, 2012 Hedge funds Quarterly 60-89 days notice	Mar. 31, 2012 Hedge funds Quarterly 90-119 days notice	Mar. 31, 2012 Hedge funds Semi-annual	Mar. 31, 2012 Hedge funds Semi-annual 60-89 days notice	Mar. 31, 2012 Hedge funds Semi-annual 120+ days notice	Mar. 31, 2012 Hedge funds Annual	Mar. 31, 2012 Hedge funds Annual 30-59 days notice	Mar. 31, 2012 Hedge funds Annual 90-119 days notice	Mar. 31, 2012 Hedge funds Annual 120+ days notice	Mar. 31, 2012 Long/short equity	Dec. 31, 2011 Long/short equity	Mar. 31, 2012 Long/short credit & distressed	Dec. 31, 2011 Long/short credit & distressed	Mar. 31, 2012 Long diversified strategies	Dec. 31, 2011 Long diversified strategies	Mar. 31, 2012 Long/short equity REIT	Dec. 31, 2011 Long/short equity REIT	Mar. 31, 2012 Long/short equity activist	Dec. 31, 2011 Long/short equity activist	Mar. 31, 2012 Long bank loan	Dec. 31, 2011 Long bank loan	Mar. 31, 2012 Private equity funds entity	Dec. 31, 2011 Private equity funds	Mar. 31, 2012 Private equity funds 1-3 years	Mar. 31, 2012 Private equity funds 3-5 years	Mar. 31, 2012 Private equity funds 5-10 years	Mar. 31, 2012 Private equity funds >10 years	Mar. 31, 2012 Multi-sector	Dec. 31, 2011 Multi-sector	Mar. 31, 2012 Energy infrastructure and services	Dec. 31, 2011 Energy infrastructure and services	Mar. 31, 2012 Distressed residential real estate	Dec. 31, 2011 Distressed residential real estate	Mar. 31, 2012 Real estate	Dec. 31, 2011 Real estate	Mar. 31, 2012 Private equity secondaries	Dec. 31, 2011 Private equity secondaries	Mar. 31, 2012 International multi-sector, Europe	Dec. 31, 2011 International multi-sector, Europe	Mar. 31, 2012 Manufacturing/Industrial	Dec. 31, 2011 Manufacturing/Industrial	Mar. 31, 2012 Healthcare	Dec. 31, 2011 Healthcare	Mar. 31, 2012 International multi-sector, Asia	Dec. 31, 2011 International multi-sector, Asia	Mar. 31, 2012 Insurance	Dec. 31, 2011 Insurance	Mar. 31, 2012 Venture capital fund	Dec. 31, 2011 Venture capital fund
Other long-term investments
Number of investments				17																															32
Largest single fund investment		$ 24.4
Other long-term investments		259.3	254.2	131.4	125.3	44.5	37.2	28.5	21.2	6.6	6.6	79.8	27.6	31.8	20.4	19.6	5.4	14.2	25.4	16.9	8.1	0.4	52.8	48.8	33.6	32.3	16.9	16.9	14.2	14.5	13.5	12.3	0.4	0.5	127.9	128.9	38.2	15.4	67.8	6.5	28.3	26.9	26.4	28.0	24.4	27.4	12.2	9.5	10.9	11.3	6.5	7.8	6.5	6.2	3.5	2.3	3.3	3.6	3.3	3.5	2.6	2.4
Unfunded Commitments		79.8	81.5																																79.8	81.5					7.7	8.2	9.4	9.9			3.3	3.3	3.6	4.0	4.9	4.7			6.4	7.0	2.7	2.6	41.3	41.3	0.5	0.5
Distributions outstanding				3.2
Redemption requests				2.5
Maximum percentage of price difference provided by pricing services	5.00%
Maximum price difference provided by pricing services	$ 1