XML 169 R149.htm IDEA: XBRL DOCUMENT
v3.6.0.2

Statutory Capital and Surplus (Details) $ in Millions | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2014 USD ($) | Dec. 31, 2016 USD ($) yr | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2017 | Dec. 31, 2013 USD ($)
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Stockholders' Equity Attributable to Parent |  | $ 3,603.3 | $ 3,913.2 |  |  |
Amount available for dividend distributions without prior approval from regulatory authorities |  | 60.0 |  |  |  |
Amount not available for dividend distribution |  | 502.5 |  |  |  |
Distributions made |  | 5.4 |  |  |  |
Fair value investments | $ 3,343.6 | 5,039.7 | 4,050.9 | $ 3,343.6 |  |
Trading Securities |  | 4,256.8 |  |  |  |
Other long-term investments |  | 323.3 | 315.8 |  |  |
OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory net income (loss) |  | 36.7 | 44.5 | (14.2) |  |
Stockholders' Equity Attributable to Parent |  | 780.6 | 755.2 |  |  |
Distributions made |  | 79.2 | 80.0 |  |  |
Unrestricted cash |  | 64.5 |  |  |  |
Other Operations |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Stockholders' Equity Attributable to Parent |  | 2,294.9 | 1,124.3 |  |  |
Unrestricted cash |  | 1,655.0 |  |  |  |
Total Capital Contributed to Subsidiary |  | 703.0 | 676.0 |  |  |
Other long-term investments |  | 66.9 |  |  |  |
Unregulated Operation |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Amount available for dividend distributions without prior approval from regulatory authorities |  | 2,300.0 |  |  |  |
Unregulated Operation | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Distributions made |  | 5.7 | 5.3 |  |  |
Unrestricted cash |  | 56.4 |  |  |  |
White Mountains Advisors LLC | Other Operations |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Unrestricted cash |  | 17.8 |  |  |  |
Star & Shield Insurance Exchange [Member] | Other Operations |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus |  | 4.4 | 5.0 |  |  |
Statutory net income (loss) |  | 20.4 | (3.5) |  |  |
Surplus required |  | 0.4 |  |  |  |
OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Dividends available, net of noncontrolling interest |  | 177.1 |  |  |  |
HG Global | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Preferred stock, value |  | $ 619.0 |  |  |  |
Ownership interest (as a percent) |  | 96.90% |  |  |  |
Preferred stock dividend rate |  | 6.00% |  |  |  |
HG Re | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus |  | $ 467.2 |  |  |  |
Collateral Held in Supplement Trust |  | 465.4 |  |  |  |
BAM | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus |  | 431.5 |  |  |  |
Statutory net income (loss) |  | 32.7 | 32.0 | 31.8 |  |
Surplus required |  | 66.0 |  |  |  |
Subsidiaries |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Stockholders' Equity Attributable to Parent |  | 562.5 |  |  |  |
One Beacon Insurance Company | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Distributions made | 700.5 |  |  |  |  |
Noncash and Part Noncash Distribution Received | 151.2 |  |  |  |  |
Split Rock [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus |  | 270.2 | 249.5 |  |  |
Statutory net income (loss) |  | $ 42.4 | 45.5 | 46.2 |  |
Percent Available for Dividend Distribution without Prior Approval from Regulatory Agency in Regulated Entities |  | 15.00% |  |  |  |
Split Rock [Member] | Regulated Operation | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus |  | $ 270.2 |  |  |  |
Amount available for dividend distributions without prior approval from regulatory authorities |  | 60.1 |  |  |  |
Distributions made |  | 25.0 |  |  |  |
Capital |  | 210.1 |  |  |  |
Statutory accounting practices earned surplus |  | 60.1 |  |  |  |
Total Capital Contributed to Subsidiary |  |  | 85.0 |  |  |
Statutory Accounting Practices, Statutory Amount Available for Contributions without Regulatory Approval |  | 31.5 |  |  |  |
White Mountains | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Distributions made |  | 60.3 |  |  |  |
Atlantic Speciality Insurance Company (ASIC) [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Surplus required |  | 107.5 |  |  |  |
Atlantic Speciality Insurance Company (ASIC) [Member] | Regulated Operation | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Amount available for dividend distributions without prior approval from regulatory authorities |  | 11.4 |  |  |  |
Distributions made |  | 26.5 |  |  |  |
Capital |  | 624.8 |  |  |  |
Statutory accounting practices earned surplus |  | $ 69.0 |  |  |  |
Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Interest rate (as a percent) |  | 8.00% |  |  |  |
basis points |  | 3.00% |  |  |  |
Preferred stocks | HG Global | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Dividends Payable |  | $ 186.2 |  |  |  |
Preferred stocks | White Mountains | HG Global | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Dividends Payable |  | 180.2 |  |  |  |
Common Equities [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Trading Securities |  | 11.9 |  |  |  |
Common Equities [Member] | Other Operations |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Trading Securities |  | 285.6 |  |  |  |
Retained earnings |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Stockholders' Equity Attributable to Parent | $ 3,010.5 | $ 2,797.2 | 3,084.9 | $ 3,010.5 |  | $ 2,801.9
Retained earnings | Split Rock [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory Accounting Practices, Statutory Capital, Percent |  | 25.00% |  |  |  |
Retained earnings | Atlantic Speciality Insurance Company (ASIC) [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory Accounting Practices, Statutory Capital, Percent |  | 10.00% |  |  |  |
U.S and Bermuda [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus |  | $ 624.8 | $ 622.3 |  |  |
Surplus Note | Unregulated Operation | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Fair value investments |  | $ 71.9 |  |  |  |
Combination One | Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Number of years of change in interest rate |  | 5 |  |  |  |
Combination Two [Member] | Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Number of years of change in interest rate | yr |  | 3 |  |  |  |
US Treasury Rate [Member] | Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Interest rate (as a percent) |  | 3.54% |  |  |  |
Fixed interest rate [Member] | Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Interest rate (as a percent) |  | 8.00% |  |  |  |
Scenario, Forecast [Member] | US Treasury Rate [Member] | Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Interest rate (as a percent) |  |  |  |  | 3.78% |

X
- Definition The amount of total capital as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47304-110998 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47080-110998 + Details
Name: | us-gaap_Capital
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding. + References Reference 1: 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)(15)(5)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(24)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(20)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: 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(15)(5)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_DividendsPayableCurrentAndNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 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 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(5)) -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 -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558 Reference 8: 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 Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 + Details
Name: | us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of long-term investments classified as other. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(f)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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_OtherLongTermInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cash outflow in the form of capital distributions and dividends to common shareholders, preferred shareholders and noncontrolling interests. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3291-108585 + Details
Name: | us-gaap_PaymentsOfDividends
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The percentage rate used to calculate dividend payments on preferred stock. + References No definition available. + Details
Name: | us-gaap_PreferredStockDividendRatePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Aggregate par or stated value of issued nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer). This item includes treasury stock repurchased by the entity. Note: elements for number of nonredeemable preferred shares, par value and other disclosure concepts are in another section within stockholders' equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_PreferredStockValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_StatutoryAccountingPracticesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of dividends that can be declared or paid without approval of regulatory agency. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(e)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
Name: | us-gaap_StatutoryAccountingPracticesStatutoryAmountAvailableForDividendPaymentsWithoutRegulatoryApproval
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country. + References Reference 1: 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)(23)(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529 + Details
Name: | us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of statutory capital required to be maintained as of the balance sheet date under prescribed or permitted statutory accounting practices. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529 + Details
Name: | us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusRequired
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators. + References Reference 1: 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)(23)(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(30)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 4.E) -URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74512-122707 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(31)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_StockholdersEquity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of investments in debt and equity securities and other forms of securities that provide ownership interests 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 2 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26626-111562 Reference 2: 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 3: 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=66010992&loc=d3e26610-111562 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 + Details
Name: | us-gaap_TradingSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount Available for Dividend Distribution without Prior Approval from Regulatory Agency in Regulated Entities + References No definition available. + Details
Name: | wtm_AmountAvailableforDividendDistributionwithoutPriorApprovalfromRegulatoryAgencyinRegulatedEntities
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount Not Available for Dividend Distribution + References No definition available. + Details
Name: | wtm_AmountNotAvailableforDividendDistribution
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition basis points + References No definition available. + Details
Name: | wtm_Basispoints
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Collateral Held in Supplement Trust + References No definition available. + Details
Name: | wtm_CollateralHeldinSupplementTrust
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Sum of net unrestricted cash, short-term investments, and fixed maturity investments as of the balance sheet date. + References No definition available. + Details
Name: | wtm_NetUnrestrictedCashShortTermAndFixedMaturityInvestments
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Noncash or Part Noncash Distribution Received + References No definition available. + Details
Name: | wtm_NoncashorPartNoncashDistributionReceived
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of years of change in interest rate + References No definition available. + Details
Name: | wtm_Numberofyearsofchangeininterestrate
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percent Available for Dividend Distribution without Prior Approval from Regulatory Agency in Regulated Entities + References No definition available. + Details
Name: | wtm_PercentAvailableforDividendDistributionwithoutPriorApprovalfromRegulatoryAgencyinRegulatedEntities
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the earned surplus as of the balance sheet date. + References No definition available. + Details
Name: | wtm_StatutoryAccountingPracticesEarnedSurplus
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Statutory Accounting Practices, Statutory Amount Available for Contributions without Regulatory Approval + References No definition available. + Details
Name: | wtm_StatutoryAccountingPracticesStatutoryAmountAvailableforContributionswithoutRegulatoryApproval
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Statutory Accounting Practices, Statutory Capital, Percent + References No definition available. + Details
Name: | wtm_StatutoryAccountingPracticesStatutoryCapitalPercent
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Total Capital Contributed to Subsidiary + References No definition available. + Details
Name: | wtm_TotalCapitalContributedtoSubsidiary
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_OnebeaconMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_OtherOperationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis=us-gaap_UnregulatedOperationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_HgGlobalBamMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

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

X
- Details
Name: | us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis=us-gaap_RegulatedOperationMember
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: |

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

X
- Details
Name: | us-gaap_DebtInstrumentAxis=wtm_SurplusNoteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_PreferredStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=wtm_WhiteMountainsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=wtm_CommonEquitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_RetainedEarningsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementGeographicalAxis=wtm_U.SandBermudaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | invest_InvestmentAxis=wtm_SurplusNoteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentRedemptionPeriodAxis=wtm_CombinationOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentRedemptionPeriodAxis=wtm_CombinationTwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=wtm_USTreasuryRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=wtm_FixedinterestrateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |