Employee Share-Based Incentive Compensation Plans (WTM Non-Qualified and Qualified Options) (Details) (Non-Qualified Options, USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2013	Dec. 31, 2011
Non-Qualified Options
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	125,000
Exercise price (in dollar per share)	$ 742
Threshold for in-the-money potential	$ 100.0
Limit on value of in-the-money options over $100 million (as a percent)	50.00%
Expense recognized		$ 0.1

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 14.F)

-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(1)(i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section F

+ Details

Name:	us-gaap_AllocatedShareBasedCompensationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of options outstanding, including both vested and non-vested options.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(i)-(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage limit of in-the-money potential value above $100 million.

+ References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsInMoneyLimitValueOver100Million
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of in-the-money potential above which a 50% limit is imposed.

+ References

No definition available.

+ Details

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

- Definition

This element represents the amended exercise price for the non-qualified options.

+ References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingAmendedExercisePrice
Namespace Prefix:	wtm_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant