XML 85 R48.htm IDEA: XBRL DOCUMENT v3.19.2

Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Kudu Investment Management, LLC
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 142.2	$ 0.0
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	114.7	0.0
DavidShield & PassportCard
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	75.0	75.0
Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	55.1	0.0
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	39.1	60.0
Kudu
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	0.0	30.7
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	426.1	165.7
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	166.1	146.1
Insurance-linked securities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	40.3	0.0
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	31.9	13.8
White Mountains
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 664.4	$ 325.6

X

- Definition

Amount of investments classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f,g))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

X

- 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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=wtm_ElementumHoldingsL.P.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

Name:	dei_LegalEntityAxis=wtm_WhiteMountainsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: