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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, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Share-based compensation arrangement by share-based payment award |  |  |  |
Accrued incentive compensation | $ 140.0 | $ 140.7 |  |
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 | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Vested (in shares) | (5,000) |  |  |
Performance Cycle From 2013 to 2015 | 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 | 140.00% |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 142.00% |  |  |
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% |  |
OneBeacon | Long Term Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Shares outstanding (in shares) | 452,519 | 449,435 | 517,470 | 493,421
Accrued incentive compensation | $ 1.6 | $ 1.4 | $ 3.4 | $ 4.0
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share | 12.00% | 13.00% | 14.00% |
OneBeacon | Range Of Performance Cycle From 2016-2018 [Member] | Long Term Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 163,150 |  |  |
Incentive compensation payable before forfeitures | $ 0.9 |  |  |
OneBeacon | Range Of Performance Cycle From 2015 to 2017 [Member] | Long Term Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 146,659 |  |  |
Incentive compensation payable before forfeitures | $ 0.7 |  |  |
OneBeacon | Range Of Performance Cycle From 2014-2016 [Member] | Long Term Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 142,710 |  |  |
Incentive compensation payable before forfeitures | $ 0.0 |  |  |
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) | 81,577 |  |  |
Incentive compensation payable before forfeitures | $ 43.1 |  |  |
forfeitures | (1,224) |  |  |
Assumed forfeitures in period amount | (0.7) |  |  |
Shares outstanding (in shares) | 80,353 | 93,654 | 111,257 | 108,605
Accrued incentive compensation | $ 42.4 | $ 57.7 | $ 44.4 | $ 54.9
White Mountains | Range Of Performance Cycle From 2016-2018 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 19,615 |  |  |
Incentive compensation payable before forfeitures | $ 5.4 |  |  |
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) | 31,795 |  |  |
Incentive compensation payable before forfeitures | $ 19.1 |  |  |
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) | 30,167 |  |  |
Incentive compensation payable before forfeitures | $ 18.6 |  |  |
Sirius Group | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Shares outstanding (in shares) | 7,315 | 10,826 | 12,607 |
Minimum | Range Of Performance Cycle From 2016-2018 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 0.00% |  |  |
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 | OneBeacon | Long Term Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 5.00% | 6.00% | 7.00% |
Maximum | Range Of Performance Cycle From 2016-2018 [Member] | W T M Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00% |  |  |
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 | OneBeacon | Long Term Incentive Plan | Performance Shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 19.00% | 20.00% | 21.00% |

X
- 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 The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + 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)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&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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Data Type: | xbrli:monetaryItemType
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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 growth in intrinsic business value per share to achieve payment. + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Targeted Performance Goal for Personnel Expressed as Percentage of Growth in Intrinsic Business Value + References No definition available. + Details
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