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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 | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014 | Dec. 31, 2014
White Mountains | Restricted Share Cliff Vesting in January 2018 |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |
Issued (in shares) |  |  | 22,078 |  |
White Mountains | Restricted Share Cliff Vesting in January 2017 |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |
Issued (in shares) |  |  | 20,400 |  |
White Mountains | Restricted Stock |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |
Beginning of period (in shares) | 68,303 | 79,011 | 80,050 | 90,341 | 90,341
Issued (in shares) | 0 | 0 | 22,078 | 20,400 |
Vested (in shares) | (1,200) | 0 | (35,025) | (31,730) |
Assumed forfeitures and cancellations | 0 | 0 | 0 | 0 |
Ending of period (in shares) | 67,103 | 79,011 | 67,103 | 79,011 | 80,050
Unamortized grant date fair value |  |  |  |  |
Unamortized grant date fair value, beginning balance | $ 21.9 | $ 20.5 | $ 13.7 | $ 16.4 | $ 16.4
Issued | 0.0 | 0.0 | 14.7 | 11.8 |
Assumed forfeitures and cancellations | 0.0 | 0.0 | 0.0 | 0.0 |
Expense recognized | (3.5) | (3.2) | (10.0) | (10.9) |
Unamortized grant date fair value, ending balance | $ 18.4 | $ 17.3 | $ 18.4 | $ 17.3 | $ 13.7
Sirius Group | Restricted Stock |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |
Ending of period (in shares) | 3,571 | 2,314 | 3,571 | 2,314 |

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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 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + 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 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://www.xbrl.org/2003/role/presentationRef -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=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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- Definition Number of share instruments newly issued under a share-based compensation 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 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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