XML 39 R47.htm IDEA: XBRL DOCUMENT
v2.4.0.6

Investment Securities (Investment gains (losses) for Level 3) (Details) (Level 3 Inputs, USD $) In Millions, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2013 | Mar. 31, 2012 | Mar. 31, 2013 Fixed maturity investments. | Dec. 31, 2011 Fixed maturity investments. | Mar. 31, 2013 Common equity securities | Dec. 31, 2011 Common equity securities | Mar. 31, 2013 Other long-term investments | Dec. 31, 2011 Other long-term investments
Investment securities |  |  |  |  |  |  |  |
Unrealized gains (losses) on investments | $ 6.5 | $ 15.5 | $ 0.2 | $ 7.8 | $ (0.1) | $ 0.5 | $ 6.4 | $ 7.2

X
- Details
Name: | us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The aggregate increase (decrease) in the market value of unsold investments whose gains (losses) are included in earnings. + References No definition available. + Details
Name: | wtm_UnrealizedGainsLossesOnInvestments
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration