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Derivatives (Forward Contract Collateral) (Details) In Millions, unless otherwise specified	Sep. 30, 2013 USD ($)	Sep. 30, 2013 JPY (¥)	Dec. 31, 2012 USD ($)	Dec. 31, 2012 JPY (¥)	Sep. 30, 2013 Forward Contracts USD ($) Rating	Sep. 30, 2013 Forward Contracts SEK	Dec. 31, 2012 Forward Contracts USD ($)	Sep. 30, 2013 Interest Rate Cap Rating	Sep. 30, 2013 A Forward Contracts Barclays USD ($)	Dec. 31, 2012 A Forward Contracts Barclays USD ($)	Sep. 30, 2013 A Forward Contracts Deutsche Bank USD ($)	Dec. 31, 2012 A Forward Contracts Deutsche Bank USD ($)	Sep. 30, 2013 A plus Forward Contracts JP Morgan USD ($)	Dec. 31, 2012 A plus Forward Contracts JP Morgan USD ($)	Sep. 30, 2013 A- Forward Contracts Goldman Sachs USD ($)	Dec. 31, 2012 A- Forward Contracts Goldman Sachs USD ($)	Sep. 30, 2013 Standard & Poor's, AA- Rating [Member] Forward Contracts HSBC USD ($)	Dec. 31, 2012 Standard & Poor's, AA- Rating [Member] Forward Contracts HSBC USD ($)	Sep. 30, 2013 Standard & Poor's, AA- Rating [Member] Forward Contracts Royal Bank of Canada USD ($)	Dec. 31, 2012 Standard & Poor's, AA- Rating [Member] Forward Contracts Royal Bank of Canada USD ($)
Derivative [Line Items]
Net amount of exposure after effect of collateral held	$ 2,200.0	¥ 217,100.0	$ 2,700.0	¥ 230,000.0	$ 16.4	105.0	$ 31.2		$ 3.1	$ 7.7	$ 5.4	$ 11.1	$ 0	$ 1.9	$ 2.5	$ 0.4	$ 4.3	$ 10.1	$ 1.1	$ 0
Uncollaterailized balance of forward currency derivatives					$ (0.1)		$ (0.1)		$ (0.1)	$ (0.1)	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
S and P Credit Ratings					21	21		21

X

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

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X

- Details

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Data Type:	xbrli:stringItemType
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X

- Definition

Represents the S&P creditworthiness heirarchy ratings used by the entity.

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
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X

- Definition

Uncollaterailized Balance of Forward Currency Derivatives

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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