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Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Restricted Share Cliff Vesting in January 2016 [Member] |  |  |
Target Performance Share activity |  |  |
Issued (in shares) |  |  | 25,720
W T M Incentive and Phantom Plan [Member] | Restricted Share Cliff Vesting in January 2017 [Member] |  |  |
Target Performance Share activity |  |  |
Issued (in shares) |  | 23,440 |
W T M Incentive and Phantom Plan [Member] | Restricted Share Cliff Vesting in January 2018 [Member] |  |  |
Target Performance Share activity |  |  |
Issued (in shares) | 23,640 |  |
White Mountains | W T M Incentive Plan | Restricted Stock |  |  |
Target Performance Share activity |  |  |
Beginning of period | 83,314 | 94,130 | 69,910
Issued (in shares) | 23,640 | 23,440 | 25,720
Vested (in shares) | (36,279) | (33,205) | (1,500)
Assumed forfeitures and cancellations (in shares) | 0 | (1,051) | 0
End of period (4) | 70,675 | 83,314 | 94,130
Unamortized grant date fair value |  |  |
Beginning balance | $ 14.3 | $ 17.0 | $ 16.8
Issued | 15.7 | 13.1 | 14.4
Assumed forfeitures and cancellations | 0.0 | (0.5) | 0.0
Expense recognized | (14.3) | (15.3) | (14.2)
Ending balance | $ 15.7 | $ 14.3 | $ 17.0
White Mountains | W T M Incentive Plan | Performance Shares |  |  |
Target Performance Share activity |  |  |
Beginning of period | 113,198 | 108,605 | 108,065
Assumed forfeitures and cancellations (in shares) | (3,857) | (3,257) | (20)
End of period (4) | 96,211 | 113,198 | 108,605
Unamortized grant date fair value |  |  |
Assumed forfeitures and cancellations | $ (0.3) | $ (0.1) | $ (0.6)
Expense recognized | (44.4) | $ (14.1) | $ (39.1)
Ending balance | $ 22.5 |  |
Range of Performance Cycle from 2011 to 2013 [Member] | Performance Shares |  |  |
Target Performance Share activity |  |  |
Vested (in shares) |  | (3,570) |
Range of Performance Cycle from 2011 to 2013 [Member] | Long Term Incentive Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Percentage of payout for minimum growth target percentage |  | 37.10% |
Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive and Phantom Plan [Member] | Restricted Stock |  |  |
Target Performance Share activity |  |  |
Assumed forfeitures and cancellations (in shares) |  | (3,040) |
Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive and Phantom Plan [Member] | Performance Shares |  |  |
Target Performance Share activity |  |  |
Assumed forfeitures and cancellations (in shares) | (3,040) |  |

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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 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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