Amount after valuation allowance of reinsurance recoverables on paid losses.

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

+ Details

Name:	us-gaap_ReinsuranceRecoverablesOnPaidLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, amounts expected of incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

+ References

+ Details

Name:	us-gaap_ReinsuranceRecoverablesOnUnpaidLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

- 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

- 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

- 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

- Definition

Represents the percentage ceded under quota share reinsurance agreement.

+ References

No definition available.

+ Details

Name:	wtm_PercentageOfQuotaShareReinsuranceAgreement
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

- Definition

Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage

+ References

No definition available.

+ Details

Name:	wtm_ReinsuranceRetentionPolicyLossRatioReinsuredriskPercentage
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

Name:	us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_CropinsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_CrophailinsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=wtm_CrophailandmultipleperilcropinsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=wtm_OnebeaconMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=wtm_CombinationOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=wtm_PropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=wtm_HealthCareMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_RangeAxis=wtm_CombinationTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Third Party Reinsurance (One Beacon) (Details) - USD ($) $ in Millions	4 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	$ 22.6	$ 22.6	$ 12.2
Reinsurance recoverable on unpaid losses	214.6	214.6	161.7
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	22.6	22.6	12.2
Reinsurance recoverable on unpaid losses	214.2	$ 214.2	$ 161.6
Crop insurance [Member] \| OneBeacon
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured		52.00%
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage		98.00%
Percentage of Quota Share Reinsurance Agreement		30.00%
Crop-hail insurance [Member] \| OneBeacon
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured		80.00%
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage		100.00%
Crop-hail and multiple peril crop insurance [Member] \| OneBeacon
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured		100.00%
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured		$ 130.0
Reinsurance Retention Policy, Amount Retained		$ 20.0
OneBeacon \| Combination One
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured		95.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation		$ 10.0
OneBeacon \| Minimum \| Property [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained		3.0
OneBeacon \| Minimum \| Health Care [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	3.0
OneBeacon \| Maximum \| Property [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained		$ 5.0
OneBeacon \| Maximum \| Health Care [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	$ 5.0
OneBeacon \| Combination Two
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured		100.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation		$ 100.0