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Employee Share-Based Incentive Compensation Plans (Performance Shares Granted Under the WTM Incentive Plan) (Details) - W T M Incentive Plan - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Mar. 31, 2020 | Dec. 31, 2019 | Mar. 31, 2019 | Dec. 31, 2018
Performance Share | Performance Cycle 2018 to 2020 |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Percentage of target payout - minimum actual |  |  | 174.00% |  |  |  |  |
Percentage of target payout - maximum actual |  |  | 180.00% |  |  |  |  |
Performance Share | Performance Cycle 2016 to 2018 |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Percentage of target payout - minimum actual |  |  |  | 139.00% |  |  |  |
Percentage of target payout - maximum actual |  |  |  | 166.00% |  |  |  |
White Mountains | Restricted Share Cliff Vesting in January 2022 [Member] |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
New grants (in shares) |  | 1,900 |  | 15,600 |  |  |  |
White Mountains | Performance Share |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Target Performance Shares Outstanding | 43,105 |  | 43,105 |  |  |  |  |
Accrued Expense | $ 16.0 |  | $ 16.0 |  |  |  |  |
Target Performance Shares Outstanding, Assumed forfeitures | (647) |  | (647) |  |  |  |  |
Accrued Expense, Assumed forfeitures | (0.2) |  | (0.2) |  |  |  |  |
Total Target Performance Shares Outstanding | 42,458 | 42,473 | 42,458 | 42,473 | 42,458 | 42,473 | 40,602 | 40,616
Total Accrued Expense | $ 15.8 | $ 30.8 | $ 15.8 | $ 30.8 | $ 9.8 | $ 43.7 | $ 23.1 | $ 31.7
New grants (in shares) | 0 | 1,900 | 14,055 | 15,600 |  |  |  |
White Mountains | Performance Share | Performance Cycle 2018 to 2020 |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Target Performance Shares Outstanding | 13,450 |  | 13,450 |  |  |  |  |
Accrued Expense | $ 8.0 |  | $ 8.0 |  |  |  |  |
White Mountains | Performance Share | Performance Cycle 2019 to 2021 |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Target Performance Shares Outstanding | 15,600 |  | 15,600 |  |  |  |  |
Accrued Expense | $ 5.9 |  | $ 5.9 |  |  |  |  |
White Mountains | Performance Share | Performance Cycle 2020 to 2022 |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Target Performance Shares Outstanding | 14,055 |  | 14,055 |  |  |  |  |
Accrued Expense | $ 2.1 |  | $ 2.1 |  |  |  |  |

X
- 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://fasb.org/us-gaap/role/ref/legacyRef -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=116856206&loc=d3e5070-113901 + Details
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Period Type: | duration

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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://fasb.org/us-gaap/role/ref/legacyRef -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=116856206&loc=d3e5070-113901 + Details
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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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- 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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- 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 Represents the lpercentage of target achieved, high end of range. + References No definition available. + Details
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- Definition Represents the percentage of target achieved, low end of range. + References No definition available. + Details
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