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Subsequent Event Subsequent Events (Details) In Millions, unless otherwise specified	12 Months Ended				1 Months Ended		2 Months Ended
	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Feb. 19, 2014 Subsequent Event WOBI	Jan. 31, 2014 Subsequent Event Star & Shield LLC USD ($)	Jan. 31, 2013 Subsequent Event Star & Shield LLC USD ($)	Feb. 19, 2014 Subsequent Event Common Stock Israel, New Shekels WOBI ILS	Feb. 19, 2014 Subsequent Event Common Stock United States of America, Dollars WOBI USD ($)	Feb. 19, 2014 Subsequent Event Preferred stocks Israel, New Shekels WOBI ILS	Feb. 19, 2014 Subsequent Event Preferred stocks United States of America, Dollars WOBI USD ($)
Subsequent Event [Line Items]
Ownership interest (as a percent)				54.00%
Purchases of common equity securities	$ 432.2	$ 365.2	$ 297.8				14.4	$ 4.1	12.7	$ 3.6
Payments to acquire interest in subsidiaries and affiliates					1.8
Purchase of Surplus Notes						$ 12.0

X

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The cash outflow associated with the acquisition of or advances to an entity that is related to it but not strictly controlled (for example, an unconsolidated subsidiary, affiliate, and joint venture or equity method investment) or the acquisition of an additional interest in a subsidiary (controlled entity).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585

+ Details

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

X

- Details

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

X

- Definition

The cash outflow to acquire common equity securities classified as trading.

+ References

No definition available.

+ Details

Name:	wtm_PaymentsToAcquireTradingCommonEquitySecurities
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Purchase of Surplus Notes

+ References

No definition available.

+ Details

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