Derivatives (Details) In Billions, unless otherwise specified	Sep. 30, 2012 USD ($)	Sep. 30, 2012 JPY (¥)	Dec. 31, 2011 USD ($)	Dec. 31, 2011 JPY (¥)
Derivatives Disclosure [Abstract]
Total guarantee value	$ 3.0	¥ 230.5	$ 3.0	¥ 233.7
Collective account values of underlying variable annuities as a percentage of the guarantee value	80.00%	80.00%	78.00%	78.00%

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

No definition available.

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