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Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Mar. 31, 2017 | Jun. 30, 2016 | Mar. 31, 2016 | Jun. 30, 2017 | Jun. 30, 2016
White Mountains | Restricted Stock |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Beginning of period (in shares) | 60,140 | 70,620 | 66,470 | 70,675 | 70,620 | 70,675
Issued (in shares) | 1,050 |  | 0 |  | 17,785 | 21,215
Vested (in shares) | (6,571) |  | 0 |  | (28,586) | (24,620)
Assumed forfeitures and cancellations | (804) |  | 0 |  | (6,004) | (800)
Ending of period (in shares) | 53,815 | 60,140 | 66,470 | 66,470 | 53,815 | 66,470
Unamortized grant date fair value |  |  |  |  |  |
Unamortized grant date fair value, beginning balance | $ 29.1 | $ 19.7 | $ 29.2 | $ 15.7 | $ 19.7 | $ 15.7
Issued | 0.9 |  | 0.0 |  | 16.7 | 16.3
Assumed forfeitures and cancellations | (0.7) |  | 0.0 |  | (3.5) | 0.2
Expense recognized | (5.8) |  | (5.0) |  | (9.4) | (8.0)
Unamortized grant date fair value, ending balance | $ 23.5 | $ 29.1 | $ 24.2 | $ 29.2 | $ 23.5 | $ 24.2
White Mountains | Restricted Share Cliff Vesting in January 2018 |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Issued (in shares) | 250 |  |  |  |  |
White Mountains | Restricted Share Cliff Vesting in January 2019 |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Issued (in shares) | 250 |  |  | 21,215 |  |
White Mountains | Restricted Share Cliff Vesting in January 2020 |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Issued (in shares) | 550 | 16,735 |  |  |  |
Sirius Group | Restricted Stock |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |
Ending of period (in shares) | 2,195 |  | 5,235 |  | 2,195 | 5,235

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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 + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -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 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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