Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Unrealized investment gains	$ (83.6)	$ 97.5	$ (294.7)
Foreign Currency Transaction Gain (Loss), Unrealized	(11.8)	11.7	(2.7)
Net unrealized investment gains (losses), pre-tax	(95.4)	109.2	(297.4)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	9.1	(4.0)	48.3
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	9.1	(4.0)	48.3
Unrealized investment gains after tax	(74.5)	93.5	(246.4)
Trading Securities, Unrealized Foreign Currenc Gain (Loss) Net of Tax	(11.8)	11.7	(2.7)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(86.3)	105.2	(249.1)
Fixed maturity investments
Investment securities
Unrealized investment gains	(8.3)	13.8	(14.6)
Foreign Currency Transaction Gain (Loss), Unrealized	(14.8)	12.7	2.1
Net unrealized investment gains (losses), pre-tax	(23.1)	26.5	(12.5)
Common equity securities
Investment securities
Unrealized investment gains	(105.5)	99.3	(257.4)
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0	(3.3)
Net unrealized investment gains (losses), pre-tax	(105.5)	99.3	(260.7)
Other long-term investments
Investment securities
Unrealized investment gains	30.2	(15.6)	(22.7)
Foreign Currency Transaction Gain (Loss), Unrealized	3.0	(1.0)	(1.5)
Net unrealized investment gains (losses), pre-tax	$ 33.2	$ (16.6)	$ (24.2)

Amount before tax of foreign currency transaction unrealized gain (loss) recognized in the income statement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109240200&loc=d3e30690-110894

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109240200&loc=d3e30700-110894

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895

+ Details

Name:	us-gaap_ForeignCurrencyTransactionGainLossUnrealized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of unrealized holding gain (loss) recognized in the income statement for 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_TradingSecuritiesUnrealizedHoldingGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The after-tax aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period.

+ References

No definition available.

+ Details

Name:	wtm_NetUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period.

+ References

No definition available.

+ Details

Name:	wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax

+ References

No definition available.

+ Details

Name:	wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesTax
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the income taxes attributable to unrealized gains (losses) on trading securities.

+ References

No definition available.

+ Details

Name:	wtm_TradingSecuritiesTaxonUnrealizedHoldingGainLossonInvestments
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The after-tax gain (loss) attributable to the foreign currency transaction gain (loss) on trading securities held at each balance sheet date, that was included in earnings in the period.

+ References

No definition available.

+ Details

Name:	wtm_TradingSecuritiesUnrealizedForeignCurrencyGainLossNetOfTax
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The tax expense (benefit) attributable to the foreign currency transaction gain (loss) on trading securities held at each balance sheet date, that was included in earnings in the period.

+ References

No definition available.

+ Details

Name:	wtm_TradingSecuritiesUnrealizedForeignCurrencyTransactionGainLossTax
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the net unrealized investment gains (losses) after income taxes.

+ References

No definition available.

+ Details

Name:	wtm_TradingSecuritiesUnrealizedHoldingGainLossonInvestmentsafterTax
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_OtherLongTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: