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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Range Of Performance Cycle From 2017-2019 |  |  |  |
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% |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00% |  |  |
Range Of Performance Cycle From 2016-2018 |  |  |  |
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 | 4.00% |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00% |  |  |
Range Of Performance Cycle From 2015-2017 |  |  |  |
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% |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, 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) | 51,285 |  |  |
Incentive compensation payable before forfeitures | $ 46.6 |  |  |
Assumed forfeitures | (770) |  |  |
Assumed forfeitures in period amount | (0.8) |  |  |
Shares outstanding (in shares) | 50,515 | 80,353 | 93,654 | 111,257
Accrued incentive compensation | $ 45.8 | $ 42.4 | $ 57.7 | $ 44.4
White Mountains | Range Of Performance Cycle From 2017-2019 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 16,680 |  |  |
Incentive compensation payable before forfeitures | $ 8.3 |  |  |
White Mountains | Range Of Performance Cycle From 2016-2018 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 16,235 |  |  |
Incentive compensation payable before forfeitures | $ 14.7 |  |  |
White Mountains | Range Of Performance Cycle From 2015-2017 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 18,370 |  |  |
Incentive compensation payable before forfeitures | $ 23.6 |  |  |
Minimum | Range Of Performance Cycle From 2017-2019 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 1.00% |  |  |
Minimum | Range Of Performance Cycle From 2016-2018 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 0.00% |  |  |
Minimum | Range Of Performance Cycle From 2015-2017 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 2.00% |  |  |
Maximum | Range Of Performance Cycle From 2017-2019 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 9.00% |  |  |
Maximum | Range Of Performance Cycle From 2016-2018 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 8.00% |  |  |
Maximum | Range Of Performance Cycle From 2015-2017 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 14.00% |  |  |

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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://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=96867065&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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