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Debt and Standby Letter of Credit Facilities (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Nov. 30, 2012 | Dec. 31, 2012 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Sep. 29, 2015 | Oct. 10, 2014 | Aug. 14, 2013 | May. 31, 2003
Debt Instrument |  |  |  |  |  |  |  |  |
Interest expense on debt |  |  | $ 18.6 | $ 15.6 | $ 16.2 |  |  |  |
Draw down of debt and revolving line of credit |  |  | 195.5 | 133.6 | 200.0 |  |  |  |
Repayments of debt, principal |  |  | $ 95.6 | 66.5 | 275.0 |  |  |  |
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate |  |  | 6.07% |  |  |  |  |  |
Interest Paid |  |  | $ 17.6 | 15.6 | 16.2 |  |  |  |
Debt |  |  | $ 442.4 | $ 343.1 |  |  |  |  |
OneBeacon Bank Facility [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  | $ 65.0 |  |  |
WTM Bank Facility |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Effective yield (as a percent) |  |  | 3.74% | 3.65% |  |  |  |  |
Interest expense on debt |  |  | $ 0.1 | $ 0.3 | $ 0.3 |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  |  | $ 425.0 |
Draw down of debt and revolving line of credit |  |  | 125.0 | 65.0 |  |  |  |  |
Line of Credit Facility, Fair Value of Amount Outstanding |  |  | 50.0 |  |  |  |  |  |
Repayments of lines of credit |  |  | 75.0 | 65.0 |  |  |  |  |
Prior W T M Bank Facility |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  |  | $ 375.0 |
One Beacon US Holdings Inc. | 2012 OBH Senior Notes |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Debt instrument at face value | $ 275.0 |  |  |  |  |  |  |  |
Percentage of par value at which debt was issued | 99.90% |  |  |  |  |  |  |  |
Issuance of debt, net of debt issuance costs | $ 272.9 |  |  |  |  |  |  |  |
Interest rate (as a percent) | 4.60% |  |  |  |  |  |  |  |
Debt issue costs | $ 2.8 |  |  |  |  |  |  |  |
Underwriting discount | $ 1.8 |  |  |  |  |  |  |  |
Effective yield (as a percent) | 4.70% |  |  |  |  |  |  |  |
Interest Expense |  |  | 13.0 |  |  |  |  |  |
Expenses related to issuance, including underwriting fees | $ 2.8 |  |  |  |  |  |  |  |
One Beacon US Holdings Inc. | 2003 OBH Senior Notes |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Guarantee fee basis points per annum on outstanding principal amount of debt (as a percent) |  |  |  |  |  |  |  |  | 2.50%
One Beacon US Holdings Inc. | Senior Notes | 2003 OBH Senior Notes |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  | 5.875% |  |  |  |  |  |  |
Debt Instrument, Repurchase Amount |  | $ 269.8 |  |  |  |  |  |  |
Aggregate purchase price of senior notes purchased and retired |  | 275.9 |  |  |  |  |  |  |
Loss on retirement of debt |  | (6.3) |  |  |  |  |  |  |
Write off of deferred issuance costs |  | $ 0.2 |  |  |  |  |  |  |
Tranzact [Member] | Tranzact Bank Facility |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Interest expense on debt |  |  | 4.0 | 0.7 |  |  |  |  |
Total commitment under revolving credit facility |  |  | 116.0 |  |  |  |  |  |
Draw down of debt and revolving line of credit |  |  | 24.0 |  |  |  |  |  |
Line of Credit Facility, Fair Value of Amount Outstanding |  |  | 104.7 |  |  |  | $ 70.0 |  |
Line of Credit Facility, Amount Outstanding |  |  | $ 104.7 | 68.7 |  |  |  |  |
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate |  |  | 4.73275% |  |  |  |  |  |
Repayments of lines of credit |  |  | $ 13.0 |  |  |  |  |  |
Debt |  |  |  | 67.4 |  |  |  |  |
MediaAlpha [Member] | MediaAlpha Bank Facility [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  | 20.0 |  |  |  |  |  |
Line of Credit Facility, Fair Value of Amount Outstanding |  |  |  | $ 0.0 |  |  |  |  |
Line of Credit Facility, Amount Outstanding |  |  | $ 15.0 |  |  |  |  |  |
Minimum | Tranzact [Member] | Tranzact Bank Facility |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
LIBOR rate |  |  | 2.50% |  |  |  |  |  |
Maximum | Tranzact [Member] | Tranzact Bank Facility |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
LIBOR rate |  |  | 4.50% |  |  |  |  |  |
Term Loan [Member] | Tranzact [Member] | Tranzact Bank Facility |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  | $ 101.0 |  |  |  |  |  |
Debt Instrument, Increase (Decrease), Other, Net |  |  | $ 31.0 |  |  |  |  |  |
Line of Credit Facility, Collateral |  |  | 70.0 |  |  |  |  |  |
Repayments of lines of credit |  |  | $ 6.5 |  |  |  |  |  |
Term Loan [Member] | MediaAlpha [Member] | MediaAlpha Bank Facility [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  | 15.0 |  |  |  |  |  |
Revolving Credit Facility [Member] | Tranzact [Member] | Tranzact Bank Facility |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  | 15.0 |  |  |  |  |  |
Revolving Credit Facility [Member] | MediaAlpha [Member] | MediaAlpha Bank Facility [Member] |  |  |  |  |  |  |  |  |
Debt Instrument |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  | $ 5.0 |  |  |  |  |  |

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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 + Details
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- Definition Discussion of whether the borrowings under the credit facility are secured or unsecured and, if secured, a description of collateral (for example, accounts receivable less than 120 days old, finished goods inventory). + 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.(b)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + 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 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 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 210.5-02.22) -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 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 + Details
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- Definition Amount of cash inflow from contractual arrangement with the lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements. + 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 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3255-108585 Reference 3: 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.(f)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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- Definition Represents guarantee fee which is equal to basis points per annum on the outstanding principal amount, paid to guarantor for unconditionally guarantying the payment of principal and interest of the debt. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition LIBOR rate + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |