XML 20 R46.htm IDEA: XBRL DOCUMENT
v2.4.0.8

Investment Securities (Investment gains (losses) for Level 3) (Details) (Level 3 Inputs, USD $) In Millions, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2014 | Mar. 31, 2013
Investment securities |  |
Unrealized gains (losses) on investments | $ 7.1 | $ 6.5
Fixed maturity investments. |  |
Investment securities |  |
Unrealized gains (losses) on investments | 0.2 | 0.2
Common equity securities |  |
Investment securities |  |
Unrealized gains (losses) on investments | 0.8 | (0.1)
Other long-term investments |  |
Investment securities |  |
Unrealized gains (losses) on investments | $ 6.1 | $ 6.4

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