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Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - White Mountains - W T M Incentive Plan - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Restricted Stock |  |  |  |
Target Restricted Share Activity |  |  |  |
Beginning of period (in shares) | 41,495 | 41,510 | 41,510 | 53,755
Issued (in shares) | 1,900 | 0 | 15,600 | 14,105
Vested (in shares) | 0 | 0 | (13,715) | (25,381)
Assumed forfeitures and cancellations | 0 | 0 | 0 | (969)
Ending of period (in shares) | 43,395 | 41,510 | 43,395 | 41,510
Unamortized grant date fair value |  |  |  |
Unamortized grant date fair value, beginning balance | $ 22.8 | $ 22.2 | $ 12.5 | $ 14.3
Issued | 1.7 | 0.0 | 14.5 | 11.4
Assumed forfeitures and cancellations | 0.0 | 0.0 | 0.0 | (0.2)
Expense recognized | (2.6) | (3.2) | (5.1) | (6.5)
Unamortized grant date fair value, ending balance | $ 21.9 | $ 19.0 | $ 21.9 | $ 19.0
Restricted Share Cliff Vesting in January 2022 |  |  |  |
Target Restricted Share Activity |  |  |  |
Issued (in shares) | 1,900 |  | 15,600 |
Restricted Share Cliff Vesting in January 2021 |  |  |  |
Target Restricted Share Activity |  |  |  |
Issued (in shares) |  | 13,450 |  |
Restricted Share Cliff Vesting in January 2020 |  |  |  |
Target Restricted Share Activity |  |  |  |
Issued (in shares) |  | 290 |  |
Restricted Share Cliff Vesting in January 2019 |  |  |  |
Target Restricted Share Activity |  |  |  |
Issued (in shares) |  | 365 |  |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + 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=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&loc=d3e5070-113901 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&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://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)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Number of share instruments newly issued under a share-based compensation 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 -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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