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Third Party Reinsurance (Sirius Group) (Details) (USD $)
In Millions, unless otherwise specified

Sep. 30, 2013

Dec. 31, 2012

Reinsurance Retention Policy [Line Items]

Reinsurance recoverable on paid losses

$ 18.8

$ 17.9

Reinsurance recoverable on unpaid losses

416.0

429.1

Sirius

Reinsurance Retention Policy [Line Items]

Reinsurance recoverable on paid losses

12.9

Reinsurance recoverable on unpaid losses

335.8

OneBeacon

Reinsurance Retention Policy [Line Items]

Reinsurance recoverable on paid losses

5.9

Reinsurance recoverable on unpaid losses

80.2

Percentage of total paid and unpaid reinsurance recoverables

90.00%

Number of ratings in A.M. Best ratings

16

Berkshire Hathaway | AM Best, A++ Rating [Member] | Sirius

Reinsurance Retention Policy [Line Items]

Reinsurance recoverables

56.0

Percentage of total paid and unpaid reinsurance recoverables

16.00%

Percentage Collateralized

0.00%

Swiss Re Group | AM Best, A+ Rating [Member] | Sirius

Reinsurance Retention Policy [Line Items]

Reinsurance recoverables

31.3

Percentage of total paid and unpaid reinsurance recoverables

9.00%

Percentage Collateralized

1.00%

Swiss Re Group | AM Best, A+ Rating [Member] | OneBeacon

Reinsurance Retention Policy [Line Items]

Reinsurance recoverables

4.6

Percentage of total paid and unpaid reinsurance recoverables

5.00%

Olympus Reinsurance Company | Standard Poors NR Rating [Member] | Sirius

Reinsurance Retention Policy [Line Items]

Reinsurance recoverables

23.0

^[1]

Percentage of total paid and unpaid reinsurance recoverables

7.00%

^[1]

Percentage Collateralized

100.00%

^[1]

Lloyds of London | AM Best, A Rating [Member] | Sirius

Reinsurance Retention Policy [Line Items]

Reinsurance recoverables

17.5

^[2]

Percentage of total paid and unpaid reinsurance recoverables

5.00%

^[2]

Percentage Collateralized

10.00%

^[2]

GIC of India | AM Best, A+ Rating [Member] | Sirius

Reinsurance Retention Policy [Line Items]

Reinsurance recoverables

$ 14.7

Percentage of total paid and unpaid reinsurance recoverables

4.00%

Percentage Collateralized

1.00%

[1]	Non-U.S. insurance entity. The balance is fully collateralized through trust agreements or funds held.
[2]	Represents the total of reinsurance recoverables due to Sirius Group from all Lloyds Syndicates.

X

- Definition

Amount after valuation allowance recoverable including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 6

-Article 7



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 20

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387



Reference 3: 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

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Balance Type:	debit
Period Type:	instant

X

- Definition

Amount after valuation allowance of reinsurance recoverables on paid losses.

+ 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

+ Details

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X

- 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

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

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X

- Definition

Percentage of total reinsurance recoverables.

+ References

No definition available.

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X

- Definition

The number of ratings in the A.M. Best rating system.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
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X

- Definition

Represents the percentage of a top reinsurer's recoverable amounts that are collateralized.

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
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X

- Definition

The respective third party reinsurers' percentage share of the total paid and unpaid reinsurance recoverables.

+ References

No definition available.

+ Details

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