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Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified |  |  | 9 Months Ended | 3 Months Ended | 9 Months Ended
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Sep. 30, 2014 | Dec. 31, 2013 | Sep. 30, 2014 Performance Share W T M Incentive Plan Performance Cycle 2011 to 2013 | Sep. 30, 2013 Performance Share W T M Incentive Plan Performance cycle 2010-2012 | Sep. 30, 2014 White Mountains Performance Share W T M Incentive Plan | Sep. 30, 2013 White Mountains Performance Share W T M Incentive Plan | Sep. 30, 2014 White Mountains Performance Share W T M Incentive Plan | Sep. 30, 2013 White Mountains Performance Share W T M Incentive Plan | Sep. 30, 2014 White Mountains Performance Cycle 2011 to 2013 Performance Share
Target Performance Share Activity |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  |  | 127,537 | 118,976 | 119,220 | 119,357 |
Shares paid or expired (in shares) |  |  |  |  | 0 | 0 | (37,130) | (47,310) |
New grants (in shares) |  |  |  |  | 0 | 250 | 45,660 | 47,170 |
Assumed forfeitures and cancellations |  |  |  |  | 0 | (6) | (213) | 3 |
Ending of period (in shares) |  |  |  |  | 127,537 | 119,220 | 127,537 | 119,220 |
Accrued Expense |  |  |  |  |  |  |  |  |
Accrued expense at the beginning of period | $ 187.5 | $ 218.3 |  |  | $ 50.5 | $ 34.9 | $ 60.2 | $ 29.4 |
Payments and deferrals |  |  |  |  | 0 | 0 | (26.7) | (11.0) |
Assumed forfeitures and cancellations |  |  |  |  | (0.2) | (0.2) | 0 | (0.2) |
Expense recognized |  |  |  |  | 6.8 | 7.1 | 23.6 | 23.6 |
Accrued expense at the end of the period | 187.5 | 218.3 |  |  | 57.1 | 41.8 | 57.1 | 41.8 |
Vesting Period |  |  |  |  |  |  | 3 years |  |
Percentage of target payout - minimum actual |  |  | 88.00% | 33.00% |  |  |  |  |
Percentage of target payout - maximum actual |  |  | 131.50% | 98.00% |  |  |  |  |
Vested (in shares) |  |  |  |  |  |  |  |  | 3,570
Additional compensation cost that would be recognized if all outstanding performance shares vested |  |  |  |  | $ 38.6 |  | $ 38.6 |  |

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- 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 Unrecognized cost of unvested share-based compensation awards. + 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 (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (a)(1) -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 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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- 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 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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