Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Investment securities
Distributions from Inactive Hedge Funds	$ 3.3
Private equity funds
Investment securities
Other long-term investments	129.8
1-3 years \| Private equity funds
Investment securities
Other long-term investments	27.7
3-5 years \| Private equity funds
Investment securities
Other long-term investments	15.6
5-10 years \| Private equity funds
Investment securities
Other long-term investments	75.9
Greater than 10 years \| Private equity funds
Investment securities
Other long-term investments	$ 10.6

This element represents the total fair market value of investments in certain entities that calculate net asset value per share (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements, but may also be used in both the balance sheet and disclosure in the same submission. This item represents alternative investments as of the balance sheet date which may include investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds, and any other investment in which investee management calculates net asset value per share as the primary metric for investment value.

No definition available.

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