Investment Securities - Net Realized and Unrealized Investment Gains and Losses - (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	$ 115.1	$ 156.8	$ 187.1
Unrealized investment gains (losses)	(313.4)	(395.2)	529.7
Net realized and unrealized investment gains (losses)	(93.0)	(380.3)	686.0
Gain (loss) on investments related to foreign currency exchange	(29.3)	(7.7)	4.0
Gross realized investment gains	129.9	212.3	214.4
Gross realized investment losses	14.8	55.5	27.3
Investments Held
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(191.5)	(246.1)	716.8
Fixed maturity investments
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	(11.9)	3.9	10.9
Unrealized investment gains (losses)	(168.4)	(42.2)	27.5
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	(1.4)	(0.1)	0.4
Unrealized investment gains (losses)	(1.1)	0.0	0.0
Common equity securities
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	0.0	0.4	137.2
Unrealized investment gains (losses)	(7.1)	14.9	(130.6)
Net realized and unrealized investment gains (losses)	(7.1)	15.3	6.6
MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	0.0	160.3	63.8
Unrealized investment gains (losses)	(93.0)	(540.6)	622.2
Net realized and unrealized investment gains (losses)	(93.0)	(380.3)	686.0
Other long-term investments
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	128.4	(7.7)	(25.2)
Unrealized investment gains (losses)	(43.8)	172.7	10.6
Total Investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(198.3)	(238.4)	716.8
Amount attributable to TPC Providers
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	6.8	(7.7)	0.0
Fixed maturity and short-term investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(182.8)	(38.4)	38.8
Less: net realized and unrealized gains (losses) on investment securities sold during the period	(2.9)	(8.4)	(8.7)
Fixed maturity and short-term investments \| Investments Held
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(179.9)	(30.0)	47.5
Common equity securities and investment in MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(100.1)	(365.0)	692.6
Less: net realized and unrealized gains (losses) on investment securities sold during the period	0.0	20.3	38.3
Common equity securities and investment in MediaAlpha \| Investments Held
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	$ (100.1)	$ (385.3)	$ 654.3

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (e)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27357-111563

+ Details

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

- Definition

Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

+ Details

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

- Definition

Amount of realized and unrealized gain (loss) on investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27405-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(c),9(a))
-URI https://asc.fasb.org/extlink&oid=126953954&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585

+ Details

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

- Definition

The net realized gain (loss) on investments sold during the period, not including gains (losses) on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, which, for cash flow reporting, is a component of proceeds from investing activities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585

+ Details

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

- Definition

Amount of realized gain (loss) on investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.3(a))
-URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

+ Details

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

- Definition

Amount of unrealized gain (loss) on investment.

+ References

+ Details

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

- Definition

Gain (Loss) On Investments, Exchange Rate Effect

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=wtm_InvestmentsHeldMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=wtm_MediaAlphaMember
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:

- Details

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

- Details

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

- Details

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

- Details

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