XML 64 R46.htm IDEA: XBRL DOCUMENT
v3.7.0.1

Third Party Reinsurance (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2017 | Dec. 31, 2016
Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid losses | $ 3.4 | $ 6.6
Reinsurance recoverable on unpaid losses | 174.7 | 172.9
OneBeacon [Member] |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance recoverable on paid losses | 3.4 | 6.6
Reinsurance recoverable on unpaid losses | $ 174.7 | $ 172.9
Property Catastrophe Program [Member] | OneBeacon [Member] |  |
Reinsurance Retention Policy [Line Items] |  |
Percentage of par value of policy reinsured | 100.00% |
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation | $ 110.0 |
Property Catastrophe Program [Member] | Minimum | OneBeacon [Member] |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance Retention Policy, Amount Retained | 20.0 |
Property Catastrophe Program [Member] | Maximum [Member] | OneBeacon [Member] |  |
Reinsurance Retention Policy [Line Items] |  |
Reinsurance Retention Policy, Excess Retention, Amount Reinsured | $ 130.0 |

X
- Definition Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 + Details
Name: | us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to and paid by the ceding insurer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 2: 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),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_ReinsuranceRecoverablesOnPaidLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Stated amount that is the upper retention limit per entity policy. + References No definition available. + Details
Name: | us-gaap_ReinsuranceRetentionPolicyAmountRetained
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of risk reinsured in excess of stated retention per company policy. + References No definition available. + Details
Name: | us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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_ReinsuranceRetentionPolicyLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of risk reinsured on insurance products per entity policy. + References No definition available. + Details
Name: | us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation + References No definition available. + Details
Name: | wtm_ReinsuranceRetentionPolicyExcessRetentionAmountReinsuredNetOfParticipation
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_OnebeaconMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsurancePolicyTypeAxis=wtm_PropertyCatastropheProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |