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Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses | $ (48.4) | $ (41.9) | $ (76.7) |
Increase (decrease) in reserves | 1,040.5 | 1,193.9 | 1,174.3 |
Accretion of fair value adjustment to net loss and LAE reserves | 1.7 | 10.6 | 8.3 |
Un-accreted fair value, adjustment to loss and loss adjustment expense reserves, before tax | 4.7 |  |  |
Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses | (23.9) | (34.5) | (46.9) |
Liability for asbestos and environmental claims, gross | 227.8 | 228.6 | 207.2 | 214.3
Liability for asbestos and environmental claims | 193.9 | 189.4 | 162.7 | 169.6
Asbestos and Environmental Loss Three Year Survival Ratio Period | 8 years | 9 years |  |
Liability for asbestos claims, net, period increase (decrease) | 11.8 | 46.4 | 10.3 |
Liabilty for environmental claims, net, period increase (decrease) | 0.8 | (0.5) | 2.0 |
OneBeacon |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses |  | (7.4) | (29.8) |
Liability for asbestos and environmental claims, gross | 1.0 |  |  |
Chile Earthquake Loss Cover | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Increase (decrease) in reserves |  |  | (13.1) |
Asbestos | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Liability for asbestos and environmental claims, gross | 207.4 | 208.2 | 185.1 | 191.9
Liability for asbestos and environmental claims | 178.7 | 174.2 | 146.2 | 151.5
Environmental | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Liability for asbestos and environmental claims, gross | 20.4 | 20.4 | 22.1 | 22.4
Liability for asbestos and environmental claims | 15.2 | 15.2 | 16.5 | 18.1
WM Solutions | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Liability for asbestos claims, net, period increase (decrease) | 13.0 | 11.0 |  |
Liabilty for environmental claims, net, period increase (decrease) | 1.0 | 0.7 |  |
Accident and Health Insurance Product Line [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses | (9.2) |  |  |
Asbestos [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses | 11.8 | 46.4 |  |
Casualty Runoff [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses |  | (31.5) |  |
Aviation Space [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses | (10.1) | (5.3) |  |
Property [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses | (16.6) | (28.4) |  |
Property Insurance Product Line [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Increase (decrease) in reserves |  |  | (41.2) |
asbestos and environmental [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Increase (decrease) in reserves |  |  | 12.3 |
Marine Energy [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses |  | (11.6) |  |
Trade Credit [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses |  | (6.8) |  |
Accident and Health | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Loss and LAE incurred relating prior year losses |  | 4.3 |  |
Professional Liability | OneBeacon |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Increase (decrease) in reserves |  |  | (11.5) |
Property and Casualty, Commercial Insurance Product Line [Member] | OneBeacon |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Increase (decrease) in reserves |  |  | (14.0) |
Other Lines of Business [Member] | OneBeacon |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Increase (decrease) in reserves |  |  | (4.3) |
Quarterly Monitoring [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Liability for asbestos claims, net, period increase (decrease) |  | 14.0 |  |
In-depth Analysis [Member] | Sirius Group |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |
Liability for asbestos claims, net, period increase (decrease) |  | $ 33.0 |  |

X
- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, before estimated recoveries from reinsurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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Balance Type: | credit
Period Type: | instant

X
- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of provision in the period for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition The amount of the other adjustment made to the claims reserves during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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X
- Definition Represents the amount of accreted fair value adjustment to loss and LAE reserves related to purchase accounting. + References No definition available. + Details
Name: | wtm_AccretedFairValueAdjustmentToLossAndLossAdjustmentExpenseReserves
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Data Type: | xbrli:monetaryItemType
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- Details
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X
- Definition Represents the survival ratio, which is the number of years that the carried A&E reserves will last assuming the current level of annual payments (based on a three-year average). + References No definition available. + Details
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- Definition Liability for Asbestos Claims, Net, Period Increase (Decrease) + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Liabilty for Environmental Claims, Net, Period Increase (Decrease) + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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X
- Definition Represents the amount of pre-tax unaccreted fair value adjustment to loss and LAE reserves related to purchase accounting. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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