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Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Jun. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015 | Dec. 31, 2015
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Common shares, issued shares | 4,578,698 |  |  | 4,578,698 |  | 5,623,735
Equity in earnings of unconsolidated affiliates, net of tax | $ 0.0 | $ 3.9 |  | $ 0.0 | $ 18.0 |
Accrued Expense |  |  |  |  |  |
Accrued expense at the beginning of period |  |  |  | 140.7 |  |
Accrued expense at the end of the period | $ 124.2 |  |  | $ 124.2 |  | $ 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%
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2015 to 2017 [Member] |  |  |  |  |  |
Target Performance Share Activity |  |  |  |  |  |
New grants (in shares) | 3,000 |  |  |  |  |
Performance Share | W T M Incentive Plan | Performance Cycle 2016 to 2018 |  |  |  |  |  |
Target Performance Share Activity |  |  |  |  |  |
New grants (in shares) | 3,400 |  |  |  |  |
White Mountains | Restricted Share Cliff Vesting in January 2019 [Member] | W T M Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Issued (in shares) | 4,150 |  |  | 25,365 |  |
White Mountains | Performance Share | W T M Incentive Plan |  |  |  |  |  |
Target Performance Share Activity |  |  |  |  |  |
Beginning of period (in shares) | 73,297 | 106,572 | 111,257 | 93,654 | 111,257 | 111,257
Shares paid or expired (in shares) |  | (8,958) |  | (36,294) | (42,959) |
New grants (in shares) | 6,400 |  |  | 22,615 | 29,195 |
Assumed forfeitures and cancellations | (160) | (3,960) |  | (438) | (3,839) |
Ending of period (in shares) | 79,537 | 93,654 | 106,572 | 79,537 | 93,654 | 93,654
Accrued Expense |  |  |  |  |  |
Accrued expense at the beginning of period | $ 32.5 | $ 28.8 | $ 44.4 | $ 57.7 | $ 44.4 | $ 44.4
Payments and deferrals |  | (5.5) |  | (41.0) | (30.4) |
Assumed forfeitures and cancellations | (0.1) | (0.8) |  | (0.2) | (0.3) |
Expense recognized | 5.0 | 30.0 |  | 20.5 | 38.8 |
Accrued expense at the end of the period | 37.4 | $ 52.5 | $ 28.8 | $ 37.4 | $ 52.5 | $ 57.7
Vesting Period |  |  |  | 3 years |  |
Additional compensation cost that would be recognized if all outstanding performance shares vested | $ 26.9 |  |  | $ 26.9 |  |
Sirius Group | Performance Share | W T M Incentive Plan |  |  |  |  |  |
Target Performance Share Activity |  |  |  |  |  |
Ending of period (in shares) | 7,315 | 10,825 |  | 7,315 | 10,825 |

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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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