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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2012
Target Performance Share activity |  |  |  |  |  |  |
Assumed forfeitures and cancellations (in shares) |  |  |  |  | 0 |  |
Accrued Expense |  |  |  |  |  |  |
Accrued expense at the end of the period | $ 218.3 |  | $ 159.0 |  |  |  |
W T M Incentive Plan |  |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  |  | 46,250 |  |
Assumed forfeitures and cancellations (in shares) |  |  |  |  | 0 |  |
Ending of period (in shares) |  |  |  |  | 72,000 |  |
Accrued Expense |  |  |  |  |  |  |
Assumed forfeitures and cancellations |  |  |  |  | 0 |  |
Expense recognized |  |  |  |  | 10.7 |  |
Vested (in shares) |  |  |  |  | (1,500) | [1] |
W T M Incentive Plan | Performance Shares [Member] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |
Options nonvested, number before forfeitures (in shares) | 122,277 |  |  |  |  |  |
Incentive compensation payable before forfeitures | 61.7 |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |  |
Ending of period (in shares) | 119,220 |  |  |  |  |  |
Accrued Expense |  |  |  |  |  |  |
Accrued expense at the end of the period | 60.2 |  |  |  |  |  |
forfeitures | (3,057) |  |  |  |  |  |
Assumed forfeitures in period amount | (1.5) |  |  |  |  |  |
W T M Incentive and Phantom Plan [Member] | Performance Shares [Member] |  |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |  |
Beginning of period (in shares) | 119,357 |  | 150,064 |  | 163,184 |  | 119,357
Shares paid or expired (in shares) | (47,310) | [2] | (68,357) | [2] | (51,131) | [2] |
Assumed forfeitures and cancellations (in shares) | (3) | [3] | (782) | [3] | (336) | [3] |
New grants (in shares) | 47,170 |  | 38,432 |  | 37,675 |  |
Ending of period (in shares) | 119,220 |  | 119,357 |  | 150,064 |  | 119,357
Accrued Expense |  |  |  |  |  |  |
Accrued expense at the beginning of period | 29.4 |  | 66.1 |  | 29.4 |  | 29.4
Payments and deferrals | (11.0) | [2] | (58.3) | [2] | 0 | [2] |
Assumed forfeitures and cancellations | (0.6) | [3] | 0.6 | [3] | (0.9) | [3] |
Expense recognized | 42.4 |  | 21.0 |  | 37.6 |  |
Accrued expense at the end of the period | 60.2 |  | 29.4 |  | 66.1 |  | 29.4
Performance Cycle From 2011 to 2013 | W T M Incentive Plan | Performance Shares [Member] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share | 10.00% |  |  |  |  |  |
Options nonvested, number before forfeitures (in shares) | 37,130 |  |  |  |  |  |
Incentive compensation payable before forfeitures | 28.2 |  |  |  |  |  |
Accrued Expense |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00% |  |  |  |  |  |
Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares [Member] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share | 8.00% |  |  |  |  |  |
Options nonvested, number before forfeitures (in shares) | 47,170 |  |  |  |  |  |
Incentive compensation payable before forfeitures | 12.1 |  |  |  |  |  |
Accrued Expense |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00% |  |  |  |  |  |
Performance Cycle From 2012 to 2014 | W T M Incentive Plan | Performance Shares [Member] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share | 8.00% |  |  |  |  |  |
Options nonvested, number before forfeitures (in shares) | 37,977 |  |  |  |  |  |
Incentive compensation payable before forfeitures | $ 21.4 |  |  |  |  |  |
Accrued Expense |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00% |  |  |  |  |  |
Performance Cycle From 2011 to 2013 | Minimum | W T M Incentive Plan | Performance Shares [Member] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |
Performance goal percentage for minimum payout | 3.00% |  |  |  |  |  |
Performance Cycle From 2011 to 2013 | Maximum | W T M Incentive Plan | Performance Shares [Member] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |
Performance goal percentage for minimum payout | 17.00% |  |  |  |  |  |
Performance Cycle From 2012 to 2014 | Minimum | W T M Incentive Plan | Performance Shares [Member] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |
Performance goal percentage for minimum payout | 2.00% |  |  |  |  |  |
Performance Cycle From 2012 to 2014 | Maximum | W T M Incentive Plan | Performance Shares [Member] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |
Performance goal percentage for minimum payout | 14.00% |  |  |  |  |  |
Performance Cycle From 2013 to 2015 | Minimum | W T M Incentive Plan | Performance Shares [Member] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |
Performance goal percentage for minimum payout | 2.00% |  |  |  |  |  |
Performance Cycle From 2013 to 2015 | Maximum | W T M Incentive Plan | Performance Shares [Member] |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |
Performance goal percentage for minimum payout | 14.00% |  |  |  |  |  |
[1] | During 2012, the Compensation Committee accelerated the vesting date for an installment of 5,000 restricted shares from January 20, 2013 to December 31, 2012.
[2] | WTM performance shares payments in 2013 for the 2010-2012 performance cycle ranged from 33% to 98% of target. WTM performance share payments in 2012 for the 2009-2011 performance cycle ranged from 147% to 155% of target. At December 31, 2012, White Mountains paid $9.9 as a partial payment for the 2010-2012 performance cycle. There were no payments made in 2011 for the 2008-2010 performance cycle; those performance shares did not meet the threshold performance goal and expired.
[3] | Amounts include changes in assumed forfeitures, as required under GAAP.

X
- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The total reduction to unamortized compensation cost for performance shares forfeited during the period. + References No definition available. + Details
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- Definition Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. + References No definition available. + Details
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Balance Type: | credit
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- Definition Carrying value as of the balance sheet date for performance shares before forfeitures. + References No definition available. + Details
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- Definition The amount of performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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- Definition This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. + References No definition available. + Details
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- Definition This element represents the reduction to accrued expense for assumed forfeitures of performance shares. + References No definition available. + Details
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- Definition This element represents the number of non-vested performance shares before forfeiture assumptions. + References No definition available. + Details
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- Definition The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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- Definition Represents the lpercentage of target achieved, high end of range. + References No definition available. + Details
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- Definition Represents the percentage growth in intrinsic business value per share to achieve payment. + References No definition available. + Details
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- Definition Represents the targeted percentage of growth in intrinsic business value per share to achieve 100% payment of outstanding performance shares. + References No definition available. + Details
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