XML 37 R50.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 | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Investment securities |  |  |  |
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments | $ 2.7 | $ (34.2) | $ 20.4 | $ (38.4)
Fixed maturity investments. |  |  |  |
Investment securities |  |  |  |
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments | (1.0) | (13.5) | 7.3 | (14.9)
Common equity securities |  |  |  |
Investment securities |  |  |  |
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments | 0.8 | (17.5) | 1.8 | (17.0)
Other long-term investments |  |  |  |
Investment securities |  |  |  |
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments | 0 | 0 | 0 | 0
Convertible fixed maturity investments |  |  |  |
Investment securities |  |  |  |
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments | $ 2.9 | $ (3.2) | $ 11.3 | $ (6.5)

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