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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) - W T M Incentive Plan - Performance Shares - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Range Of Performance Cycle 2019 to 2021 |  |  |  |
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 | 7.00% |  |  |
Payout for maximum growth target percentage | 200.00% |  |  |
Range Of Performance Cycle 2018 to 2020 |  |  |  |
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 | 6.00% |  |  |
Payout for maximum growth target percentage | 200.00% |  |  |
Range Of Performance Cycle From 2017 to 2019 |  |  |  |
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 | 5.00% |  |  |
Payout for maximum growth target percentage | 200.00% |  |  |
White Mountains |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 43,120 |  |  |
Incentive compensation payable before forfeitures | $ 44.3 |  |  |
Assumed forfeitures (shares) | (647) |  |  |
Assumed forfeitures in period amount | (0.6) |  |  |
Shares outstanding (in shares) | 42,473 | 40,616 | 50,515 | 80,353
Accrued incentive compensation | $ 43.7 | $ 31.7 | $ 45.8 | $ 42.4
White Mountains | Range Of Performance Cycle 2019 to 2021 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 15,600 |  |  |
Incentive compensation payable before forfeitures | $ 6.0 |  |  |
White Mountains | Range Of Performance Cycle 2018 to 2020 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 13,450 |  |  |
Incentive compensation payable before forfeitures | $ 10.5 |  |  |
White Mountains | Range Of Performance Cycle From 2017 to 2019 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 14,070 |  |  |
Incentive compensation payable before forfeitures | $ 27.8 |  |  |
Minimum | Range Of Performance Cycle 2019 to 2021 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 2.00% |  |  |
Minimum | Range Of Performance Cycle 2018 to 2020 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 2.00% |  |  |
Minimum | Range Of Performance Cycle From 2017 to 2019 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 1.00% |  |  |
Maximum | Range Of Performance Cycle 2019 to 2021 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 12.00% |  |  |
Maximum | Range Of Performance Cycle 2018 to 2020 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 10.00% |  |  |
Maximum | Range Of Performance Cycle From 2017 to 2019 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 9.00% |  |  |

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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 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 growth in intrinsic business value per share to achieve payment. + References No definition available. + Details
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