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Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Jun. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Dec. 31, 2015
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Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Total Accrued Expense | $ 76.9 |  | $ 95.7 |  |  |
White Mountains | Performance Share | W T M Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 51,085 |  |  |  |  |
Accrued Expense | $ 38.4 |  |  |  |  |
Target Performance Shares Outstanding, Assumed forfeitures | (766) |  |  |  |  |
Accrued Expense, Assumed forfeitures | (0.6) |  |  |  |  |
Total Target Performance Shares Outstanding | 50,319 | 50,575 | 80,353 | 79,537 | 73,297 | 93,654
Total Accrued Expense | $ 37.8 | $ 31.2 | $ 42.4 | $ 37.4 | $ 32.5 | $ 57.7
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2015 to 2017 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 18,370 |  |  |  |  |
Accrued Expense | $ 20.6 |  |  |  |  |
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2016 to 2018 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 16,235 |  |  |  |  |
Accrued Expense | $ 12.1 |  |  |  |  |
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2017 to 2019 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 16,480 |  |  |  |  |
Accrued Expense | $ 5.7 |  |  |  |  |

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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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Period Type: | instant

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- 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
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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
Period Type: | instant

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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 This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. + References No definition available. + Details
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Period Type: | instant

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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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- Details
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