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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2015 USD ($) multiplier shares | Dec. 31, 2014 USD ($) shares | Dec. 31, 2013 USD ($) shares
Unamortized grant date fair value |  |  |
Beginning of period | $ 108.1 |  |
End of period (4) | $ 141.7 | $ 108.1 |
Performance Shares | Range of Performance Cycle from 2011 to 2013 [Member] |  |  |
Unamortized grant date fair value |  |  |
Vested (in shares) | shares |  | 3,570 |
Performance Shares | White Mountains |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Multiplier for determining payout, low end of range | multiplier | 0 |  |
Multiplier for determining payout, high end of range | multiplier | 2 |  |
Performance cycle period | 3 years |  |
Performance Shares | W T M Incentive and Phantom Plan [Member] | Range Of Performance Cycle From 2014-2016 [Member] |  |  |
Target Performance Share activity |  |  |
Assumed forfeitures and cancellations(2)(3) | shares | (3,040) |  |
Performance Shares | W T M Incentive and Phantom Plan [Member] | Range of Performance Cycle from 2011 to 2013 [Member] |  |  |
Unamortized grant date fair value |  |  |
Performance goal percentage for minimum payout | 88.00% |  |
Payout for maximum growth target percentage | 131.50% |  |
Performance Shares | W T M Incentive and Phantom Plan [Member] | Range of Performance Cycle from 2010 to 2012 [Member] |  |  |
Unamortized grant date fair value |  |  |
Performance goal percentage for minimum payout |  | 33.00% |
Payout for maximum growth target percentage |  | 98.00% |
Performance Shares | W T M Incentive Plan | Range Of Performance Cycle From 2014-2016 [Member] |  |  |
Unamortized grant date fair value |  |  |
Payout for maximum growth target percentage | 200.00% |  |
Performance Shares | W T M Incentive Plan | White Mountains |  |  |
Target Performance Share activity |  |  |
Beginning of period | shares | 113,198 | 108,605 | 108,065
Shares paid or expired(1) | shares | (42,958) | (33,730) | (43,100)
New grants | shares | 29,828 | 41,580 | 43,660
Assumed forfeitures and cancellations(2)(3) | shares | (3,857) | (3,257) | (20)
End of period (4) | shares | 96,211 | 113,198 | 108,605
Unamortized grant date fair value |  |  |
Beginning of period | $ 44.4 | $ 54.9 | $ 26.5
Shares paid or expired(1) | (30.8) | (24.5) | (10.1)
Assumed forfeitures and cancellations(2)(3) | (0.3) | (0.1) | (0.6)
Expense recognized | 44.4 | 14.1 | 39.1
End of period (4) | 57.7 | $ 44.4 | $ 54.9
Unamortized grant date fair value | $ 22.5 |  |

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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 grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target 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 -Subparagraph (c)(2)(iii)(1) -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 The amount of performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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- Definition The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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