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Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 USD ($) shares | Mar. 31, 2020 USD ($) shares | Jun. 30, 2019 USD ($) shares | Jun. 30, 2020 USD ($) multiplier shares | Jun. 30, 2019 USD ($) shares
W T M Incentive Plan | Performance Cycle 2016 to 2018 |  |  |  |  |
Accrued Expense |  |  |  |  |
Percentage of target payout - minimum actual |  |  |  |  | 139.00%
Percentage of target payout - maximum actual |  |  |  |  | 166.00%
W T M Incentive Plan | Performance Cycle 2018 to 2020 |  |  |  |  |
Accrued Expense |  |  |  |  |
Percentage of target payout - minimum actual |  |  |  | 174.00% |
Percentage of target payout - maximum actual |  |  |  | 180.00% |
White Mountains |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Multiplier for Determining Payout, Low End of Range | multiplier |  |  |  | 0 |
Accrued Expense |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Multiplier for Determining Payout, High End of Range | multiplier |  |  |  | 2 |
Share-based Compensation Arrangement by Share-based Payment Award, Performance Cycle Period |  |  |  | 3 years |
White Mountains | W T M Incentive Plan |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |
Vesting Period |  | 3 years |  |  |
Target Performance Share Activity |  |  |  |  |
Beginning of period (in shares) | shares | 42,458 | 42,473 | 40,602 | 42,473 | 40,616
Shares paid or expired (in shares) | shares | 0 |  | 0 | (14,070) | (13,715)
New grants (in shares) | shares | 0 |  | 1,900 | 14,055 | 15,600
Assumed forfeitures and cancellations | shares | 0 |  | (29) | 0 | (28)
Ending of period (in shares) | shares | 42,458 | 42,458 | 42,473 | 42,458 | 42,473
Accrued Expense |  |  |  |  |
Accrued expense at the beginning of period | $ 9.8 | $ 43.7 | $ 23.1 | $ 43.7 | $ 31.7
Payments and deferrals | 0.0 |  | 0.0 | (27.7) | (18.1)
Assumed forfeitures and cancellations | 0.0 |  | (0.2) | 0.4 | 0.0
Expense recognized | 6.0 |  | 7.9 | (0.6) | 17.2
Accrued expense at the end of the period | 15.8 | $ 9.8 | $ 30.8 | 15.8 | $ 30.8
Additional compensation cost that would be recognized if all outstanding performance shares vested | $ 17.3 |  |  | $ 17.3 |

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- Definition Amount of cost not yet recognized for nonvested award under share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=116856206&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://fasb.org/us-gaap/role/ref/legacyRef -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=116856206&loc=d3e5070-113901 + Details
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