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Derivatives (Details) In Billions, unless otherwise specified	Dec. 31, 2013 USD ($)	Dec. 31, 2013 JPY (¥)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Notional amount	$ 1.9	¥ 203.6
Collective account values of underlying variable annuities as a percentage of the guarantee value	104.00%	104.00%

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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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

X

- Details

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X

- Definition

Represents the collective account values of underlying variable annuities as a percentage of the guarantee value of the associated variable annuity reinsurance liability.

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant