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Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015 | Dec. 31, 2015
Share-based compensation arrangement by share-based payment award |  |  |  |  |
Common shares, issued shares | 4,963,921 |  | 4,963,921 |  | 5,623,735
Equity in earnings of unconsolidated affiliates, net of tax | $ 0.0 | $ 6.8 | $ 0.0 | $ 14.1 |
Accrued Expense |  |  |  |  |
Accrued expense at the beginning of period |  |  | 140.7 |  |
Accrued expense at the end of the period | $ 106.3 |  | $ 106.3 |  | $ 140.7
Performance Share | W T M Incentive Plan | Performance Cycle 2013 to 2015 |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |
Common shares, issued shares | 5,000 |  | 5,000 |  |
Accrued Expense |  |  |  |  |
Percentage of target payout - minimum actual |  |  | 140.00% |  |
Percentage of target payout - maximum actual |  |  | 142.00% |  |
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member] |  |  |  |  |
Accrued Expense |  |  |  |  |
Percentage of target payout - minimum actual |  |  |  |  | 88.00%
Percentage of target payout - maximum actual |  |  |  |  | 131.50%
White Mountains | Performance Share | W T M Incentive Plan |  |  |  |  |
Target Performance Share Activity |  |  |  |  |
Beginning of period (in shares) | 111,387 | 108,545 | 96,358 | 113,295 | 113,295
Shares paid or expired (in shares) | (36,294) | 0 | (36,294) | (34,001) |
New grants (in shares) | 0 | 750 | 16,215 | 29,878 |
Assumed forfeitures and cancellations | 908 | (19) | (278) | 104 |
Ending of period (in shares) | 76,001 | 109,276 | 76,001 | 109,276 | 96,358
Accrued Expense |  |  |  |  |
Accrued expense at the beginning of period | $ 71.3 | $ 25.7 | $ 57.7 | $ 44.4 | $ 44.4
Payments and deferrals | (41.3) | 0.0 | (41.3) | (24.9) |
Assumed forfeitures and cancellations | 0.9 | 0.0 | 0.3 | 0.5 |
Expense recognized | 1.3 | 3.1 | 15.5 | 8.8 |
Accrued expense at the end of the period | 32.2 | $ 28.8 | $ 32.2 | $ 28.8 | $ 57.7
Vesting Period |  |  | 3 years |  |
Additional compensation cost that would be recognized if all outstanding performance shares vested | $ 27.2 |  | $ 27.2 |  |
Sirius Group | Performance Share | W T M Incentive Plan |  |  |  |  |
Target Performance Share Activity |  |  |  |  |
Ending of period (in shares) | 4,428 | 7,851 | 4,428 | 7,851 |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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- Definition This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.12) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 9 -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 11 -Article 7 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (a)(1) -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 were forfeited 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)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- 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://www.xbrl.org/2003/role/presentationRef -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=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 The amount of performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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- Definition The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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