XML 77 R60.htm IDEA: XBRL DOCUMENT
v3.22.0.1

Significant Transactions - MediaAlpha (Details) - USD ($) $ / shares in Units, $ in Millions |  |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 23, 2021 | Oct. 30, 2020 | Feb. 26, 2019 | Dec. 31, 2019 | Dec. 31, 2021 | Dec. 31, 2020
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |
Realized gain from transaction |  |  |  | $ 67.5 |  |
MediaAlpha |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |
Proceeds from dividends received |  | $ 55.0 |  |  |  |
Ownership interest (as a percent) |  | 35.00% |  |  | 28.00% |
Fully diluted ownership interest (as a percent) |  | 32.30% |  |  | 25.70% |
Investment owned (in shares) |  | 20,500,000 |  |  | 16,900,000 |
Share price (in USD per share) |  |  |  |  | $ 15.44 | $ 39.07
Investment owned, balance |  |  |  |  | $ 261.6 | $ 802.2
MediaAlpha | IPO |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) |  | 3,600,000 |  |  |  |
Sale of stock, consideration received in transaction |  | $ 63.8 |  |  |  |
MediaAlpha | Secondary Offering |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) | 3,600,000 |  |  |  |  |
Sale of stock, consideration received in transaction | $ 160.3 |  |  |  |  |
MediaAlpha |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |
Proceeds from dividends received |  |  | $ 89.3 |  |  |
Realized gain from transaction |  |  | $ 67.5 |  |  |
Ownership interest (as a percent) |  |  | 48.30% |  |  |
Fully diluted ownership interest (as a percent) |  |  | 42.00% |  |  |
Assets, fair value |  |  | $ 114.7 |  |  |
Unrealized investment gain |  |  | 114.7 |  |  |
Total realized and unrealized gain on sale of minority stake |  |  | $ 182.2 |  |  |
MediaAlpha | Secondary Offering |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) | 8,050,000.00 |  |  |  |  |
Sale of stock, price per share (in USD per share) | $ 46.00 |  |  |  |  |
Sale of stock, price per share net of underwriting fees (in dollars per share) | $ 44.62 |  |  |  |  |

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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258 + Details
Name: | us-gaap_AssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The difference between the carrying value and the sale price of an investment. A gain would be recognized when the sale price of the investment is greater than the carrying value of the investment. This element refers to the Gain included in earnings and not to the cash proceeds of the sale. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(3)(a)) -URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)) -URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(b)(9)(a)) -URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227 Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(b)(7)(c)) -URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227 + Details
Name: | us-gaap_GainOnSaleOfInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition For investments which are quantified by principal amount, the principle balance held at close of period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 50 -Paragraph 6 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column B)) -URI http://asc.fasb.org/extlink&oid=122147990&loc=d3e611197-123010 + Details
Name: | us-gaap_InvestmentOwnedBalancePrincipalAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Balance held at close of period in number of shares. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 50 -Paragraph 6 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column B)) -URI http://asc.fasb.org/extlink&oid=122147990&loc=d3e611197-123010 + Details
Name: | us-gaap_InvestmentOwnedBalanceShares
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
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_OtherSignificantNoncashTransactionsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Dividends received on equity and other investments during the current period. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 16 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3337-108585 + Details
Name: | us-gaap_ProceedsFromDividendsReceived
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Cash received on stock transaction after deduction of issuance costs. + References No definition available. + Details
Name: | us-gaap_SaleOfStockConsiderationReceivedOnTransaction
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The number of shares issued or sold by the subsidiary or equity method investee per stock transaction. + References No definition available. + Details
Name: | us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction. + References No definition available. + Details
Name: | us-gaap_SaleOfStockPricePerShare
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Price of a single share of a number of saleable stocks of a company. + References No definition available. + Details
Name: | us-gaap_SharePrice
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the equity in unrealized gains or losses from investments in unconsolidated affiliates. + References No definition available. + Details
Name: | wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Equity Method Investment, Fully Diluted Ownership Percentage + References No definition available. + Details
Name: | wtm_EquityMethodInvestmentFullyDilutedOwnershipPercentage
Namespace Prefix: | wtm_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Realized Gain From Transaction + References No definition available. + Details
Name: | wtm_RealizedGainFromTransaction
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Sale of Stock, Price Per Share, Net of Underwriting fees + References No definition available. + Details
Name: | wtm_SaleOfStockPricePerShareNetOfUnderwritingFees
Namespace Prefix: | wtm_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | srt_OwnershipAxis=wtm_MediaAlphaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsidiarySaleOfStockAxis=us-gaap_IPOMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsidiarySaleOfStockAxis=wtm_SecondaryOfferingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_MediaAlphaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |