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Investment Securities (Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
NR | Preferred Stock | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value Assets	$ 70.7	[1]
Discount Yield (as a percent)	7.80%	[1]
NR | Security Owned Not Readily Marketable, Name [Domain] | Multiple of GAAP book value
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value Assets	36.8
Book value multiple (as a percent)
BBB- | Agency CMBS | Broker Pricing
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value Assets	87.3
AAA | Agency CMBS | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Inputs Constant Prepayment Rate	5.00%
Fair value input, swap spread rate	0.70%
AA | Agency CMBS | Broker Pricing
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value Assets	$ 22.0
[1] As of September 30, 2013, asset type consists of one security

X

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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X

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

X

- Definition

Book Value Multiple to Calculate Fair Value

+ References

No definition available.

+ Details

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

X

- Definition

Represents the constant prepayment rate, used as an input to measure fair value.

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents the swap spread input used to measure fair value.

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant