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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based compensation arrangement by share-based payment award |  |  |  |
Accrued incentive compensation | $ 141.7 | $ 108.1 |  |
Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
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% |  |  |
Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
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 | 9.00% |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00% |  |  |
Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares |  |  |  |
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% |  |  |
Performance Cycle From 2012 to 2014 | W T M Incentive and Phantom Plan [Member] | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 91.00% |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 145.50% |  |  |
White Mountains | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 98,678 |  |  |
Incentive compensation payable before forfeitures | $ 59.2 |  |  |
forfeitures | (2,467) |  |  |
Assumed forfeitures in period amount | (1.5) |  |  |
Shares outstanding (in shares) | 96,211 | 113,198 | 108,605 | 108,065
Accrued incentive compensation | $ 57.7 | $ 44.4 | $ 54.9 | $ 26.5
White Mountains | Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 29,828 |  |  |
Incentive compensation payable before forfeitures | $ 8.3 |  |  |
White Mountains | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 32,557 |  |  |
Incentive compensation payable before forfeitures | $ 12.9 |  |  |
White Mountains | Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 36,293 |  |  |
Incentive compensation payable before forfeitures | $ 38.0 |  |  |
Sirius Group | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Shares outstanding (in shares) | 7,997 | 10,351 | 10,615 |
Minimum | Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 2.00% |  |  |
Minimum | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 3.00% |  |  |
Minimum | Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 2.00% |  |  |
Maximum | Range Of Performance Cycle From 2015 to 2017 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 14.00% |  |  |
Maximum | Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 15.00% |  |  |
Maximum | Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares |  |  |  |
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=6415400&loc=d3e5070-113901 + 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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