Investment Securities (Investment Holdings) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fixed maturity investments, at fair value	$ 4,784.3us-gaap_TradingSecuritiesDebt	$ 5,030.5us-gaap_TradingSecuritiesDebt
Including Short Term Fixed
Investment securities
Duration of Fixed Maturities	2 years
US Government and agency obligations
Investment securities
Trading Securities, Cost	184.7	365.5
Net foreign currency gains (losses)	3.6	(2.5)
Fixed maturity investments, at fair value	188.1	362.5
Trading Securities, Unrealized Holding Gain	0.1	0.5
Trading Securities, Unrealized Holding Loss	(0.3)	(1.0)
Debt securities issued by corporations
Investment securities
Trading Securities, Cost	2,221.3	2,330.7
Net foreign currency gains (losses)	49.8	(14.3)
Fixed maturity investments, at fair value	2,311.2	2,347.2
Trading Securities, Unrealized Holding Gain	45.2	44.0
Trading Securities, Unrealized Holding Loss	(5.1)	(13.2)
Municipal obligations
Investment securities
Trading Securities, Cost	82.0	18.3
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	83.2	17.9
Trading Securities, Unrealized Holding Gain	1.4	0
Trading Securities, Unrealized Holding Loss	(0.2)	(0.4)
Mortgage-backed and asset-backed securities
Investment securities
Trading Securities, Cost	1,811.1	2,027.3
Net foreign currency gains (losses)	25.7	(5.3)
Fixed maturity investments, at fair value	1,840.9	2,014.5
Trading Securities, Unrealized Holding Gain	7.6	2.4
Trading Securities, Unrealized Holding Loss	(3.5)	(9.9)
Foreign government, agency and provincial obligations
Investment securities
Trading Securities, Cost	274.6	444.2
Net foreign currency gains (losses)	(2.7)	(4.8)
Fixed maturity investments, at fair value	275.1	439.9
Trading Securities, Unrealized Holding Gain	4.2	3.7
Trading Securities, Unrealized Holding Loss	(1.0)	(3.2)
Preferred stocks
Investment securities
Trading Securities, Cost	79.6	79.9
Net foreign currency gains (losses)	0.1	(0.2)
Fixed maturity investments, at fair value	85.8	84.8
Trading Securities, Unrealized Holding Gain	6.1	5.1
Trading Securities, Unrealized Holding Loss	0	0
Fixed Income Investments [Member]
Investment securities
Trading Securities, Cost	4,653.3	5,265.9
Net foreign currency gains (losses)	76.5	(27.1)
Total fixed maturity investments	4,784.3	5,266.8
Trading Securities, Unrealized Holding Gain	64.6	55.7
Trading Securities, Unrealized Holding Loss	(10.1)	(27.7)
Excluding Short Term Investments
Investment securities
Duration of Fixed Maturities	2 years 3 months 18 days
OneBeacon Runoff
Investment securities
Fixed maturity investments reclassified to assets held for sale (1)		$ (236.3)

Name:	us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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

- 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 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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=6871852&loc=d3e26626-111562

Reference 3: 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 4: 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 5: 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

+ Details

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

- 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 50

-Paragraph 9

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563

Reference 3: 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

+ Details

Name:	us-gaap_TradingSecuritiesUnrealizedHoldingGain
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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 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

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

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

Reference 4: 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

+ Details

Name:	us-gaap_TradingSecuritiesUnrealizedHoldingLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Duration of fixed maturity investments in years

+ References

No definition available.

+ Details

Name:	wtm_Durationoffixedmaturityinvestmentsinyears
Namespace Prefix:	wtm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

- Definition

Represents the fair value of trading debt securities re-classified to assets held for sale.

+ References

No definition available.

+ Details

Name:	wtm_TradingSecuritiesDebtReclassifiedToAssetsHeldForSale
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=wtm_ExcludingShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=wtm_IncludingShortTermFixedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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