XML 54 R43.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | $ 7.4 | $ 0.7 | $ 21.0 | $ 265.0
Trading securities, Tax on Realized Holding Gain (Loss) on Investments | (0.6) | (0.1) | (3.6) | (45.1)
Trading securities realized holding gain (loss) on investments after tax | 6.8 | 0.6 | 17.4 | 219.9
Foreign Currency Transaction Gain (Loss), Realized | (0.6) | 0.0 | (0.2) | 0.0
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax | 0.0 | 0.0 | 0.0 | 0.0
Trading securities, realized foreign currency gain (loss) net of tax | (0.6) | 0.0 | (0.2) | 0.0
Net realized investment gains, pre-tax | 6.8 | 0.7 | 20.8 | 265.0
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | (0.6) | (0.1) | (3.6) | (45.1)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 6.2 | 0.6 | 17.2 | 219.9
Fixed maturity investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 0.6 | 0.3 | (0.4) | 2.0
Foreign Currency Transaction Gain (Loss), Realized | 1.2 | 0.0 | 2.7 | 0.0
Net realized investment gains, pre-tax | 1.8 | 0.3 | 2.3 | 2.0
Short-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | (0.1) | 0.2 | (0.1) | 0.4
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment gains, pre-tax | (0.1) | 0.2 | (0.1) | 0.4
Common equity securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 4.9 | 0.2 | 18.5 | 262.6
Foreign Currency Transaction Gain (Loss), Realized | 5.4 | 0.0 | 6.0 | 0.0
Net realized investment gains, pre-tax | 10.3 | 0.2 | 24.5 | 262.6
Other long-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 2.0 | 0.0 | 3.0 | 0.0
Foreign Currency Transaction Gain (Loss), Realized | (7.2) | 0.0 | (8.9) | 0.0
Net realized investment gains, pre-tax | $ (5.2) | $ 0.0 | $ (5.9) | $ 0.0

X
- Definition Amount before tax of foreign currency transaction realized gain (loss) recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=64836518&loc=d3e30690-110894 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64836518&loc=d3e30700-110894 + Details
Name: | us-gaap_ForeignCurrencyTransactionGainLossRealized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of gain (loss) realized from securities classified as trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 + Details
Name: | us-gaap_TradingSecuritiesRealizedGainLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax + References No definition available. + Details
Name: | wtm_NetRealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesNetofTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities + References No definition available. + Details
Name: | wtm_TaxonNetRealizedGainLossandForeignCurrencyGainLossonTradingSecurities
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax + References No definition available. + Details
Name: | wtm_TradingSecuritiesRealizedForeignCurrencyGainLossNetofTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Trading Securities, Realized Gain (Loss), Total + References No definition available. + Details
Name: | wtm_TradingSecuritiesRealizedGainLossTotal
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Trading Securities Realized Holding Gain (Loss) on Investments after Tax + References No definition available. + Details
Name: | wtm_TradingSecuritiesRealizedHoldingGainLossonInvestmentsafterTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax + References No definition available. + Details
Name: | wtm_TradingSecuritiesTaxonRealizedForeignCurrencyTransactionGainLossTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Trading securities, Tax on Realized Holding Gain (Loss) on Investments + References No definition available. + Details
Name: | wtm_TradingsecuritiesTaxonRealizedHoldingGainLossonInvestments
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_OtherLongTermInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |