Number of shares that have been repurchased and retired during the period.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29, 30

-Article 5

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Balance Type:	na
Period Type:	duration

- Definition

Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).

+ References

+ Details

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

- Definition

Total cost of shares repurchased divided by the total number of shares repurchased.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 30

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Definition

The number of additional shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Significant Transactions (Share Repurchases) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Significant Transactions
Additional authorized repurchase of common shares	545,496		545,496
Value of common shares repurchased and retired during the period		$ 26.4	$ 79.8	$ 517.4
Average price per share		$ 528.00	$ 564.00	$ 499.00
Common Stock
Significant Transactions
Stock Repurchased and Retired During Period, Shares	0	50,000	141,535	1,037,191
Combination One
Significant Transactions
Value of common shares repurchased and retired during the period				107.6
Average price per share				$ 494.00
Combination One \| Common Stock
Significant Transactions
Stock Repurchased and Retired During Period, Shares			140,000	217,801
Combination Two \| Tender Offer
Significant Transactions
Value of common shares repurchased and retired during the period				$ 408.6
Average price per share				$ 500.00
Combination Two \| Common Stock
Significant Transactions
Stock Repurchased and Retired During Period, Shares			1,535
Combination Two \| Common Stock \| Tender Offer
Significant Transactions
Stock Repurchased and Retired During Period, Shares				816,829
Combination Three \| Common Stock
Significant Transactions
Stock Repurchased and Retired During Period, Shares				2,561