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Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2020 |  |  |
Target Performance Share activity |  |  |
Issued (in shares) | 17,485 |  |
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2019 |  |  |
Target Performance Share activity |  |  |
Issued (in shares) | 250 | 24,615 |
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2018 |  |  |
Target Performance Share activity |  |  |
Issued (in shares) | 250 | 750 | 23,640
White Mountains | W T M Incentive Plan | Restricted Stock |  |  |
Target Performance Share activity |  |  |
Beginning of period | 70,620 | 70,675 | 83,314
Issued (in shares) | 17,985 | 25,365 | 23,640
Vested (in shares) | (28,846) | (24,620) | (36,279)
Assumed forfeitures and cancellations (in shares) | (6,004) | (800) | 0
End of period | 53,755 | 70,620 | 70,675
Unamortized grant date fair value |  |  |
Beginning balance | $ 19.7 | $ 15.7 | $ 14.3
Issued | 16.3 | 20.2 | 15.7
Assumed forfeitures and cancellations | (3.5) | (0.3) | 0.0
Expense recognized | (18.2) | (15.9) | (14.3)
Ending balance | $ 14.3 | $ 19.7 | $ 15.7
Sirius Group | W T M Incentive Plan | Restricted Stock |  |  |
Target Performance Share activity |  |  |
Beginning of period | 5,235 | 9,205 |
End of period | 2,195 | 5,235 | 9,205

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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=96867065&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=96867065&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=96867065&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=96867065&loc=d3e5070-113901 + Details
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