XML 104 R51.htm IDEA: XBRL DOCUMENT
v3.3.0.814

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2015 | Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Fixed maturity investments, at fair value | $ 2,602.2 | $ 2,422.0
U.S. Government and agency obligations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 99.6 | 105.4
Trading Securities, Unrealized Holding Gain | 0.2 | 0.1
Trading Securities, Unrealized Holding Loss | 0.0 | (0.3)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 99.8 | 105.2
Debt securities issued by corporations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 998.6 | 1,162.0
Trading Securities, Unrealized Holding Gain | 8.7 | 13.1
Trading Securities, Unrealized Holding Loss | (1.5) | (3.4)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 1,005.8 | 1,171.7
Municipal obligations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 145.8 | 81.0
Trading Securities, Unrealized Holding Gain | 1.8 | 1.4
Trading Securities, Unrealized Holding Loss | (0.8) | (0.2)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 146.8 | 82.2
Mortgage-backed and asset-backed securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 1,261.2 | 967.5
Trading Securities, Unrealized Holding Gain | 6.4 | 2.8
Trading Securities, Unrealized Holding Loss | (2.2) | (2.4)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 1,265.4 | 967.9
Foreign government, agency and provincial obligations |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 1.0 | 11.5
Trading Securities, Unrealized Holding Gain | 0.3 | 0.3
Trading Securities, Unrealized Holding Loss | 0.0 | (1.0)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 1.3 | 10.8
Preferred Stock |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 78.3 | 78.3
Trading Securities, Unrealized Holding Gain | 4.8 | 5.9
Trading Securities, Unrealized Holding Loss | 0.0 | 0.0
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 83.1 | 84.2
Fixed Income Investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or amortized cost | 2,584.5 | 2,405.7
Trading Securities, Unrealized Holding Gain | 22.2 | 23.6
Trading Securities, Unrealized Holding Loss | (4.5) | (7.3)
Net foreign currency gains (losses) | 0.0 | 0.0
Total fixed maturity investments including assets held for sale | $ 2,602.2 | $ 2,422.0

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 Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading. + References No definition available. + Details
Name: | us-gaap_TradingSecuritiesCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investments in debt securities classified as trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26626-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),1) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26610-111562 + Details
Name: | us-gaap_TradingSecuritiesDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of unrealized holding gain before loss recognized in the income statement for investments in trading securities. + 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=26872669&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
Name: | us-gaap_TradingSecuritiesUnrealizedHoldingGain
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of unrealized holding loss before gain recognized in the income statement for investments in trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 Reference 2: 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=26872669&loc=d3e20235-122688 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
Name: | us-gaap_TradingSecuritiesUnrealizedHoldingLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the fair value of trading debt securities including assets classified as held for sale. + References No definition available. + Details
Name: | wtm_TradingSecuritiesDebtIncludingAssetsHeldForSale
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The net amount of unrealized gains (losses) from foreign currency transaction adjustments related to trading securities. + References No definition available. + Details
Name: | wtm_TradingSecuritiesNetUnrealizedForeignCurrencyGainLoss
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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

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

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

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

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

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